Titan (Warhammer 40,000)
"Day of wrath, day of anger
will dissolve the world in ashes,
as foretold by David and the Sibyl.
Great trembling there will be
when the Judge descends from heaven
to examine all things closely..."
- – Dies Irae
"There's always a bigger fish"
- – Ancient Terran Proverb
"GO GO POWER RANGERS!!"
- – Sanctified, official Adeptus Titanicus choir
A Titan is a general term used by the Imperium for all things that are ridiculously MASSIVE and carry HUGE FUCKING RAPE GUNS that can blow the fuck out of the opposing side. The bigger ones are humanoid Giant Robots.
And they say the Tau are the only mecha-using weeaboos in the setting. Hypocrites. *BLAM* Heresy!
Most races own a titan of some sort. Each race's Titans reflect the design philosophy behind the rest of their armies. So Imperial Titans are walking Cathedrals with Gothic architecture with guns poking out of every crevice, Chaos Titans look like a doom metal rock concert made of spikes and evil, Ork Titans are cartoony and cobbled together out of odds and ends also with guns poking out of every crevice, Tyranid Bio-Titans are essentially (even) more toothy and (even) more tentacly forms of kaiju, Eldar Titans are sleek wraithbone slendermen...that incidentally move like him too.
None of the fluff writers seem to have a consistent idea of how big a Titan is (other than that it's big); Graham McNeil says an Imperator is 43 metres tall, Dan Abnett says an Imperator is over 140 metres tall, and the cover of the graphic novel Titan II: Vivaporius shows a smaller Warlord with access ladders on its guns suggesting each barrel is the size of a house, meaning the Titan itself would be over half a kilometre tall. However, as Titans have to be shipped to where they are going to be used, they can't be a (very) substantial size relative to Battleships (the largest ship available to carry them); we can't be certain of Battleship sizes *either*, but best guesses place them at 8-12k long. Combined with the fundamental scaling issue that Titans can't fly but Attack Craft (commonly 70-100 meters) can, so Titans would simply never be fielded if they were substantially larger, and 43 meters becomes far and away the most reasonable of those three guesses. Though some source says the Imperator/Warmonger is 80 meters tall, matching a common pattern of Fury interceptor's length. Given how Titans are transported, this is also a reasonable size. That, by the way, is taller than a Boeing 747-8 if it were stood on its nose. Thanks to Adeptus Titanicus, we now have official heights for the Warhound (17.08m), the Reaver (25.21m) and the Warlord (32.76m). These heights do not include any carapace weapons.
- 1 Imperial Titans
- 2 Orky Titans
- 3 Eldar Titans
- 4 Necron Titans
- 5 Bio-Titans
- 6 Tau Titans
- 7 Fielding Titans
- 8 EMERGENCY WARNING
- 9 Gallery
- 10 External Links
Imagine a city that hates you. Now give it legs to walk on and weapons that could level another city, then fill it with more things that hate you. You now have a rough idea of a Titan.
They are quite often referred to as god-machines due to their ability to blow the fucking shit out of anything in their way. They are so fucking huge that infantry can't do anything to them with their tiny ass guns. These titans can unleash unlimited amounts of RAPE via their MASSIVE FUCKING GUNS that are mounted on their arms and sometimes on their shoulders. Well, actually there are guns fucking EVERYWHERE on a titan, so when you see one you are quite fucked. They also have void shields which makes them pretty much invulnerable to whatever shit you can throw at it. Although the way Epic rules work, anything, even infantry, can shoot at it enough to overload the Void Shields and bring them down, letting other Warmachines pummel the actual Titan. In 4th-7th ed 40k, Void Shields are each AV12, requiring anti-vehicle weapons to bring down each shield. And in 8th, they're simply a special Invulnerable Save that can even block Mortal Wounds, although the Save degrades depending on how badly damaged the Titan is.
Like all machines made by the Adeptus Mechanicus more complex than a toaster, Titans have machine-spirits that reflect their nature. Meaning Titans have machine spirits full of RAGE that want to Rip and tear everything it sees. To combat this, Titans are controlled by individuals called Principes (singular Princeps) who possess extremely strong wills which can override the machine-spirit's bloodthirsty nature. However, if the Princeps is not careful in synchronization, then he goes insane and his mind is instead consumed by the Titan's machine spirit which then goes berserk and destroys fucking everything it doesn't like, ala Evangelion.
Imperial Titans are old. As a rule. No exceptions. These things are so old that they make some Eldar Titans look young by comparison. That old. The main reason that these titans are so bloody ancient is that the Imperium often lacks the knowledge, skill, or resources to build them anywhere but Mars, Ryza, and apparently Lucius. In the good old days, every other Forge World had a Titan Legion. The second reason they are so god-dang geriatric is that when Titans get killed, they are usually so big they don't get killed dead. This means that the Mechanicus can haul the thing back to the Titan's home world and fix it up again. (Not wholly true; there is a small number of highly highly-censured tech-priests that make, and even design, new Titans. However, the work is so nearly outright heresy that people don't talk about it much.) Regardless, there may be newly built Titans (as we see on Forge World Graia in the Space Marine game), but they each take at least centuries to build, and even then, only at the largest Forge Worlds with specialized Manufactorums to build them.
Oh yeah, we forgot. Imperial Titans have home worlds that run the upkeep for a whole Legion of titans of various sizes and classes. A single titan can be deployed by itself, though small detachments of titans are more common, typically the whole legion will "walk" together to end a threat. Because, for the Imperium, if we need to send one titan, we might as well send thirty-plus titans just to be sure that that severe of a threat gets taken care of. Like every other branch of the Imperium, Titan Legions all have their own internal cultures and practices ; some have a clan like social structure, some are ruthlessly meritocratic, some operate only one class of titan, others are crewed only by women, some are crewed by gene-engineered clones of crew from before the Heresy. The Mechanicus is apparently fine with all divergence, so long as the titans are cared for properly and the legions remain loyal to Mars. Titan Legions, especially Princeps, are also notable exceptions from traditional Martian sensibilities regarding augmentation. Whilst crew members will, in the line of duty, end up getting augmentations or enhancements (either due to injury or to aid in their duties), they are not expected to completely cyberise themselves, as the Mechanicus holds that the union of Human mind and Titan machine spirit is pretty damn close to the Holy Synthesis they seek. Mechanicus techtheologians even beleive that the link with Titans works precisely because the pilots are not heavily augmented.
A big part of Titans are their Machine Spirit. They are amongst some of the oldest and most sophisticated in the Imperium, sometimes verging close to sentient at times. The link between Princeps and Machine Spirit also acts as a storage buffer, keeping a part of each Princeps onboard the link. If a princeps dies whilst plugged in, their spirit can sometimes be absorbed into the Titan itself - much better than being daemon chow in the warp.
Speaking of daemons, during the Horus Heresy, several of the Titan Legions defected to Horus, giving Chaos its very own supply of Titans. Most of them are relatively similar to their loyalist counterparts but some enterprising members of the Dark Mechanicum and the Word Bearers experimented with creating actual Chaos Titans, either by having the princeps be possessed or infecting the Titan's sentience with a warp entity. A very small number were created during the Heresy but more were created after.
This unique walker is small enough (at "only" 9 meters) to be piloted by a single person, but is still staggeringly deadly. They were originally designed during the Dark Age of Technology to help with colonization, but had been repurposed for war by the Age of Strife when several members of the AdMech discovered feudal worlds whose leaders were willing to provide military assistance in exchange for the Mechanicus' aid in maintaining them. Although they are sometimes overshadowed by the full-size Titans, the Adeptus Mechanicus still deploys them as skirmishers and flankers for their Titans, and as they can be fielded in higher numbers, Knight Houses are potent forces in their own right.
There are a bunch of different types of Knights, detailed on their page. All of them have a reasonable points cost (by Titan standards, anyway) and also wield Strength 14 chainswords or giant power fists that can pick up and throw the wrecks or corpses of any vehicles or monstrous creatures that they kill. Have fun.
The smallest traditional Titan class the Imperium has but it's still massive. To put things into perspective, the Warhound is around 17 meters at rest. Metal Gear REX from the MGS series is around 13 meters high, just 4m (12 feet) short of a standard warhound. It looks like a dinosaur with no tail. It carries smaller weapons such as a massive megabolter which is like a minigun that fires off tank shells, or a Huge ass LAZAR that rips your tanks a new one like they were made of cardboard. Or maybe throw a really big flamethrower on it. Can also be used to scout behind enemy lines and reveal itself behind a bush-CREEEEEEEEEEEEEEED!
The Legio Audax were known for fielding nothing but Warhounds, using pack tactics to take down much larger opponents.
Bigger than the Warhound (just over 25m at rest), so therefore causes more RAPE, this titan can wield an absolutely HUGE powerfist for fucking Wraithlords, Defilers, etc. It can also carry a massive hellfire missile launcher for triple lehzar cannons and even a fucking Gatling Blaster. They don't tend to get depicted or given much time in fluff compared to the other 3, possibly as the pack tactics of the warhounds and supersized death machine vibe of Warlord and Imperators tend to seem more impressive.
It's worth nothing that in the Great Crusade, the Iron Hands took down an empire which favoured what were basically AT-ATs, which were described as equal in size to Reaver Titans. Let that sink in: the Star Wars universe's longtime definition of a Huge War Machine is on par with the second out of four tiers in the 40K universe... and the machine which finally one-upped it still doesn't match the Imperator.
It's really REALLY fucking big, almost 33m tall without its carapace weapons and one of the more common huge fucking titans that carry all the weapons you can imagine from megabolters to huge lazer and devastator cannons that should've been mounted on some kind of battleship. The Imperium seems to have almost as many of these as it has Reavers. Modified or custom built titans are known to exist such as the ANGRY MARINE TITAN with its HUGE chain fist, even moar massive Chain fist, oh and it launches Angry Marines and Land Raiders, fucking awesome rite? Now has an awesome new model which costs more than some used cars.
The first all-new Titan Model that is not either a Warlord, Reaver or Warhound, the Warbringer is a medium Battle Titan halfway between a Warlord and a Reaver. The Nemesis configuration refers to it mounting a Warlord-class Quake Cannon as its primary weapon, allowing it to be used as a dedicated siege platform and Anti-Titan weapon. Expect a non-Nemesis Warbringer sometime in the future.
HOLY SHIT IT WEARS CASTLES!!! Over 9000 times bigger and heavier than a Warlord Titan and too many massive fucking guns to count. It needs a whole ship for itself to get around. Unfortunately the Imperium doesn't have a lot of 'em anymore since those chaos assholes stole the majority of 'em.. They said that it's the largest thing to walk on land, because anything larger would produce its own gravity well. Oddly enough, it mounts a Ryza-pattern Plasma Annihilator, yet most of these things were supposedly made on Mars. Then again, they might predate differing planetary patterns of weapon. More likely, since all data from all Forge Worlds is put on Mars, the Martians simply used what they were given by Ryza.
TL;DR They're what would happen if Cherno Alpha was made by Mormons
If you feel compelled to represent one of these monsters in a game, the best way to do so is to make the entire 6x4 (or whatever that translates to in metric units) board into the titan's back, load it up with massive fortifications, and fight a cityfight/planetary assault on it.
Other Imperial Titan Classes
While the Warhound, Reaver and Warlord are the most common Titan classes seen in the Imperium, at the time of the Great Crusade and Horus Heresy there were others. Some, such as the Eclipse and Nightgaunt were once just Warlord variants that FW decided to make new types. Others like the Carnivore and Komodo are entirely unique designs never before seen in fluff. A few are particularly noteworthy:
First is the Apocalypse Class, which is to the Imperator what the Imperator is to the Warlord and is the biggest Titan of all. Though there are no images of it, it would most likely be a skyscraper-sized monster that would make anything that saw it shit their pants. Eisenhorn once described seeing one at a triumph on a hive world, saying that it looked like whole sections of the hive moving. The second is the three-legged Punisher-class Titan, which was used during the War of the Beast. Third is the Warlord Sinister Class, of which only 25 were made during the Great Crusade. While the Eldar make exclusive use of psychic Titans, this is the only known Imperial giant psychic death machine. The Rapier class Scout Titan was even smaller and faster than a warhound, mounting only one weapon and comparitively minimal armour and shielding. They were already a rarity in the closing days of the Heresy and the design seems to have been discontinued.
The Legio Fureans came up with the Reviler class, somewhere between a Warhounds and Reaver.
"By the Fell Gods and the destiny of warp, by the death of the False Emperor and the dying of the stars, we bring to you, Warmaster Abaddon, Beloved of Chaos, Despised of Man, this tribute. For now these last days are the final fires burning, the black flames that consume a galaxy, the storms of the warp that drown out life, the End Times and the dawn of a galaxy of Chaos. We swear fealty to the Gods of Chaos and their herald, Abaddon the Despoiler, with this tribute that it might strike fear into the followers of the Corpse-Emperor and that through it they may see the true face of death..."
- – The Castigator Titan, on itself
The Castigator-Class Autonomous Bipedal Weapons Platform is an STC Titan believed to be the design all Imperial Titans are derived from (a claim supported by the Titan itself, which referred to all other Imperial Titans derisively as "pale imitations made by ignorant children"), and like everything else made during of the Dark Age of Technology, it is to all other Titans what the God-Emperor of Mankind is to a Space Marine by comparison (the Father of all human Titans). It was a towering giant of white and silver, with a faceless head except for its gleaming red eyes. Despite being far larger than any other Titan in existence (save the Temple Gargant) it proved to be far faster and more agile (standing completely upright instead of hunched over like 40k titans) than its bulk would suggest thanks to its unique locomotion system, which featured synthetic muscles supplemented by an automatic repair system. Its weapons were no less advanced, consisting of a Titan Power Fist and a heavily modified rotary cannon of uncertain origin (it fired Daemons). (Some theorize that this was what inspired the Angry Marines to equip their Titans with Angry Marine Launchers and Land Raider Launchers, but nobody has the courage to confirm if this is the case.) Unlike any of the other Titans, it has no crew and is instead operated solely through the action of an artificial intelligence.
- SPOILER -
This turned out to be a very big problem for the Imperium, as the AI in question was advanced enough to fall to Chaos and went so far as to make a pact with the Ruinous Powers, although the book is a bit ambiguous about that too as Justicar Alaric claimed that it was a daemon that had found itself trapped inside the STC during the time the planet it was buried in was lost in the warp, and that it was in there so long it actually started to believe it was the AI. Only the efforts of an expeditionary force of Tech Guard and a single squad of Grey Knights were enough to destroy it, though fragments of its STC database was collected and stored by the Mechanicus shortly afterward to be reviewed at a later time.
No matter how much you may want to see this metal beast side with the Big E, it's most likely not gonna happen. 'Cause you know, Chaos.
The Orks use titans called Gargants, made out of scrap metal, wood, stolen stuff, and SHEER ORKINESS. They are supposedly effigies of the ork gods, but we all know the mek that built the first Gargant just wanted a bigger, killier thing than that other ork over there.
Obviously, the Orks have no standard pattern of build, but Gargants are almost always
humanoid Orkoid, wield over-sized guns, have a huge close combat weapon, and are insanely hard to take down (even compared to the other races' Titans!). The bigger ones usually move on treads, the smaller ones usually waddle. Either way, these things are slow. But who cares, because they have enough guns to blast the enemy apart, and enough other Gargants to box them in as well!
They have Power Fields, which are so finicky that they can't be repaired in battle, which sucks, but at least they have them.
Ironically, these are the most realistic titans in 40k. Unlike the rest of the titans, these either have massively proportioned feet or use treads to deal with the problem of sinking into the ground, and are very bottom heavy in order to avoid concentrating the weight of the machine on the thinnest part of the structure. If anything, the Orks should have the most impractical titan designs, not the least. Not really though, just the most low-tech solution.
Stompa: The smallest Gargant type. While Games Workshop would like you to think otherwise, the Stompa is bigger than a Mega-Dred but smaller than the current "Stompa" model, which is presently a small Gargant (don't get all smart and technical, we know the irony there). They come in bunches, and have no shields whatsoever. Their main armament is usually a gatling gun or stolen tank cannon, and then a giant close combat weapon. They eat infantry, buildings, smaller Titans, and tanks for breakfast, but only in close combat.
Gargant: Orky Titans roit an' proppa. They are more in line with the "Stompa" rules GW and Imperial Armor put out, plus a little extra. Lots of armor, lots of staying power, lots of dakka (but not enough). Usually has a better close-combat statline than any other Titan counterpart except the 'nids. Due to the utter lack of design consistency, the weaponry varies roughly from stolen Earthshaker cannons to giant gatling guns or massive friggin' laser beams, plus a gazillion other little guns so the Gargant can fire at everything at once. One configuration has so many guns on it that the entire crew cannot fire them at once. And they somehow generate Supa-rokkits out of thin air.
Great Gargant: The Great Gargant in Space Marine: Epic was what is today simply called a Gargant, and was armed with a Mega-Kannon, a Super Lifta-Droppa and a ridiculous
trouser cannon Belly Gun. The latter fired a giant iron ball which wrecked the hell out of anything short of another Titan and had rules allowing it to roll through multiple targets!
Mega Gargant: Here the line between giant robot and moving fortress blur. Where the Great Gargant is like a skyscraper, this thing actually is a fortress on treads. With a population in excess of a small city, these machines are always moved on huge treads which can literally crush Baneblades beneath them. Slab-sided and covered with gun nests and extra armor patches, this large block-like fortress is home to thousands upon thousands of grot riggers, running to and fro. Powered by salvaged space ship reactors or dangerous Ork-made engines of questionable physical legality, these gods of destruction have more guns and bigger guns than any other race will mount on a thing of equivalent size. And this is a small mountain. Bristling with turrets, artillery cannons, rocket bays, launch hangers for aerial craft, high-caliber gatling guns, flak turrets, lightning lasers, autocannons, and ONE GIANT MEGA-CANNON, each Mega Gargant is not just as tough as a block of titanium, it can evaporate you through sheer volume of shot. Also, it has the obligatory close combat weapon, which is something easy to move around (like a hammer, axe, or buzzsaw) because it will be so large that the Gargant just may not be able to move it much.
Like the Imperator, the Mega Gargant originated in Titan Legions where it had the dubious honour of having the most complicated rules of any model in the game, requiring the player keep track of dozens of Hit Locations (which could each be destroyed or the subject of a fire that could spread), the Krew (and there were different types of Krew), the Steam Counters, the commander's Shoutin' Counters to actually make the Gargant do anything... and after all that, one was destroyed in a battle report where a Space Marine side with only a basic Warlord Titan won by 105 VPs to 5. This appears to have been about the time White Dwarf decided to switch to narrative, rather than blow-by-blow, battle reports to make bullshitting the results easier.
Not all Gargants are created equal! For other Gargant Variants, see the Gargants page!
Well, yeah back to the Titan: It's the smallest Eldar Titan, and it's also the most useful Titan, as it's both fast and agile, unlike some I could mention.
The Eldar field these war machines in pairs - they are piloted by Eldar twins with a strong psychic bond to enable a greater level of awareness and cohesion to the fighting unit. For a race who are dying out and don't breed much you'd think they'd figure out how to just network two titans together or just use the radio rather than building their doctrine around psychic twins but frankly the Eldar titans could only be more anime if they were powered by magic and could only be piloted by angsty teenagers.
These agile monsters are considered as scout Titans due to their speed and agility.
It's armed with a Pulsar, which is pretty much made to tackle enemy Titans, like this bitch here. It also has an Eldar Missile Launcher, a rapid firing missile launcher. And FINALLY, it has Sonic Lance, and it's a large flame template infantry killer, which even makes space marines look like pussies.
It also gets Titan Holofields, which works like reverse 4++ against hits (basically forcing your opponent to roll D6 for each hit and completely negating it on 1-3), which, mind you, stacks with cover saves your titan can get (by, say, hiding behind the building quarter its size, bonus points if it's a 3+ cover fortification), and in case you really want to make it even tougher your farseer may cast a Fortune on it - now you may wish your opponent good luck going through three successive 4+ saves to just touch this thing.
Well, short story, pretty much your basic, Eldar killing machine, and it's the SMALLEST of the Eldar Titans.
The other titan available to the Eldar in IA11. Loaded up with all kinds of fun customizable toys. From the Phantom Pulsar which doubles the standard number of pulsar shots, the Heat Lance which turns titans to molten slag without too much difficulty, the Phantom D-Cannon which throws out an SD 10" pie plate at AP2 and does D3 STRUCTURE POINTS of damage, a power glaive which allows the phantom to take down pretty much any other titan in CC. And it's protected by the same bullshit holofields Revenant does, backed by AV13 and insane amount of hull points.
Old Epic rules included a Psyker version of the Phantom, armed with a big powerfist and a giant Psycannon. It has not been seen since then.
While no record of Titan-scale war machines exist in Imperial Archives, it should be duly noted that it is well within the reach of Necron technology to create them, and they most likely do exist. The only other race with knowledge on this matter would be the Eldar, who we are sure have already purged the memories of such constructs from their minds to rid themselves of the PTSD.
While many fans speculate over what a Necron Titan would look like, most agree it would be a Necron version of the Iron Giant; i.e. an up-sized, unkillable Necron Lord with guns aplenty. Necrons have an advantage that Titans (or similar units) from other races doesn't have: they're more resistant to Graviton weaponry due to their hyper-advanced technology. Their ships and weapons routinely ignore other laws of physics, why not ignore the Square Cube Law?
A machine suspected to be the Necron Titan looked like a worm. (Well, what would you expect from race that most time spends under the surface?) They are called Tomb Stalkers, and are strange beasties that walk and talk like monstrous creatures, but supposedly can take down Warhounds. This doesn't make them Titan-class vehicles in the strictest sense, but it's certainly up there in terms of vehicular homicide (ba-dum-tssh).
Something that could perhaps be a true "Necron Titan" (or at least the closest that have been seen to have so far, it's more the size of a Wraithknight really) appeared in the old Necromunda comic Kal Jerico: Above & Beyond. It begins with Kal's mother Jena Orechiel basically abducting him from Necromunda's underhive and whisking him away from his life of gun-slinging bounty-hunter shenanigans. She took him on a mission to the Space Hulk Kronos which had recently re-emerged from the Warp and where there supposedly rested a powerful alien weapon she wanted destroyed which, after a fun journey through the Space Hulk, turned out to be a giant Necron called The Setekh. Obviously the thing wakes up cranky and starts killing the shit out of Orechiel's retinue as well as the numerous Deathwatch Space Marines some asshole rival Inquisitor named Malva brought with him while taking zero damage from their attempts to fight back (at this point Orechiel comments that defeating it wouldn't be possible even with an entire Space Marine chapter behind them). The only ones to barely escape the hulk alive were Kal, his half-sister and his mother (oh, and her pet Kroot) who commandeered the Deathwatch ship and ordered it to perform Exterminatus on the Space Hulk which was heavily damaged by the ensuing barrage of cyclonic torpedoes and thrown back into the Warp. It's important to note this was before the changes to the Necron fluff so it's likely it never even happened. A shame because fielding this thing in one's Necron army would probably be hilarious.
Another possibility of a Necron Titan happened during the Medusa V campaign where the Necrons tried to awaken a huge Tomb Spyder like construction called a Crypt Stalker. Unfortunately it got its ass handed to it by a bunch of Chaos Titans. However, since this was from a time where the Necrons were actually cool silent Terminator-death-machines and the C'tan weren't reduced to overgrown Pokemon, it has likely been shelved alongside the Setekh.
The tyranids' equivalent of titans are predictably giant versions of other tyranids, following the style of the army in the same way the others do. As in they're giant monstrosities that will either run at you and eat your face off or wield guns that shoot things that do. Obviously, they fall under the Gargantuan Creature rules, rather than any superheavy rules. There are three kinds that currently have models, the relatively small Hierodule, the flying Harridan, and the FUCKHUEG Hierophant. Another exists, in the fluff, a couple more, one called the Dominatrix, which has an Epic-scale model but no rules for it. There's also the even less tangible Hydraphant, which is either a misidentified Epic Hierophant, an altogether new titan or a Dominatrix without the synapse organism, depending on whom you ask. Either way, the Hydraphant is the largest of the Tyranid bio-Titans. They are also, naturally, utterly impossible due to supporting huge heavy bodies on very thin spindly legs(who gives a shit?! I want one!).
For much of the Tau's history, they never had Titans or any equivalent to them. This is worth mentioning here because they thought that the idea of any civilization that had the resources and technology to build something like a Titan would surely not be so absurd as to actually waste them in such an impractical manner by building one when creating numerous smaller war machines would be much more efficient. They laughed at the first humans to tell them of Titans, assuming that it was only Imperial propaganda to intimidate their enemies. Then the Damocles Crusade made them turn their blue skin several shades paler as they realized that the gue'la were crazy enough to actually build them.
They suffered horribly at the hands of Imperial titans, but for some time they still considered titans too impractical to be worth building. The Tau utterly failed to understand the kind of wars the Imperium gets into. When you're up against a defensive line 12 miles deep, or an Ork horde large enough to make Soviet Russia blush, what you need is not 20,000 of something cost effective, but something that can just destroy large swaths of the enemy as fast as possible. In this kind of, admittedly uncommon (which is why Titans are not commonly deployed) situations, winning tactical victories is far more important that the most efficient strategic production practices. Building many smaller machines is useless if only one huge machine can wade though the fight to win the important battles. Besides, the Imperium of Man has such a powerful industrial base that they can (or at least could until they forget how) afford the cost of building multiple Titan sized units without even putting a dent in their more cost effective weapon production. To put another way, modern destroyers may be more cost effective in many situations then battleships, but sometimes you really need a battleship.
So when the Tau faced off against the Titans they needed some ways to counter them. Mantas were in the same mass class as Titans, and had big enough railguns to threaten them, and while almost invulnerable to most Titan class weapons (since they aren't designed against flying targets, even ones big enough to be mistaken for real estate) they were too vulnerable to massed anti-tank weaponry (like Lascannons) in turn, and a bit too valuable to lose cavalierly. So they ended up manufacturing a version of the Tiger Shark bomber equipped with Manta-scale railguns. For a time, it seemed to work at forcing the Imperium to be more careful about deploying Titans against the Tau, since a single flight-wing of these Tiger Shark attackers can rip up a formation of Titans and replacements for those bombers can be (relatively) easy to manufacture compared to the Titans it threatens.
Adside: Unfortunately this does not make a lot of sense, since if you think about it, the Titan's void shields and ship-level thickness of armor, plus the escorting massed anti-air support from the Mechanicus and Imperial Guard forces, pretty much made them impervious to anything. But GW has a massive hard-on for making things difficult for the Imperium, so it's hard to imagine a situations where attackers alone can drive back a Titan, unless one of two things, or both, are true. A: the armor on a Titan is like a tank, and thinner on the top, which is possible; or B: The Tau are using WWII-era "let's make that city disappear"-size bomber flights and assorted fighter support, swapping the carpet bombing for the more-accurate heavy ground-attack aircraft. Funnily enough, this is what actually happened in the real world, only replace "titan" with "battleship" and you have a perfect example for the air-dominance of modern naval combat. Although with the introduction of anti-ship ballistic missiles such as the DF-21d, and the high cost and maintenance, the relevancy and practicality of Aircraft Carriers are really stretching in the 21st century.
Back to the story at hand: due to the increasing number of Imperial Knights equipped with anti-aircraft weapons and Tyranid gargantuan creatures the Tau have been facing during their expansions, it has become clear that the tried-and-true method of spamming Tiger Shark attackers and Mantas is no longer sufficient to properly take down these new threats: Mantas were already too valuable to risk 1:1 casualty ratios, and the attacker wings cannot be mobilized or deployed fast enough for rapid deployment against unexpected war machines and themselves are vulnerable to lower-altitude anti-aircraft weaponry. In order to have a chance against these opponents, the Tau needed something that could go toe-to-toe with these towering monstrosities, or at least bombard them from long range while still being tough enough to withstand being a massive bullet magnet itself. In a move that would have previously seemed foolhardy and wasteful to try, the Tau just said "Fuck it!" and decided to take a page from Pacific Rim, building their own Titan in order to fight everyone else's.
The result was the KX-139 Ta'Unar Supremacy Armour: the first Titan-class Battlesuit ever made. Designed specifically to counter other races attempts to reclaim territory lost to the Tau expansion campaigns, this machine was designed around being a mobile Titan-hunter, heavily armed but under-armored. For its secondary arm weapons it can equip either a slightly-gimped-Multimelta-esque Fusion Eradicator (which more than overcompensates by being Heavy 5) for popping vehicles, TEQs, and monstrous creatures, or an Ion Cannon that can fire 6 normal shots, or 3 super-shots per turn, both of which are superb for massacring heavy infantry. But the real party is in the back, as the core weapons systems are mounted from an extended backpack/carapace/shoulder mounting point. The back-mounted Pulse Ordnance Multi-Driver is a 3-barreled naval gun system that slams naval-grade ordinance downrange at naval distances, and can be configured to fire all three guns at one point for massive damage, or in a spread-out pattern with explosive payloads. It can also strap one singular fuckoff gun in the form of the Heavy Rail Cannon Array, instead of three smaller fuckoff guns, or forgo direct fire entirely by mounting a Nexus Meteor Missile System to spam rockets like it's little brother (more on that later). Defensively, it has Knight-class armor that stands alongside the likes of other races' smaller Titans, and a powerful Barrier Shield that, while most effective against ranged weapons, can protect against close combat attacks. In consideration for the kind of threats the Ta'Unar will be facing, the Barrier Shield can focus all of it's energy toward one area, intentionally sacrificing all other defensive power to intercept a perceived "Deathblow" and greatly reduce the inevitable impact. This gamble will blow out the shield for a short while, but it allows the Ta'Unar to withstand (once) as much damage as it can dish out, especially since it's not as structurally fortified as other Titans, even for its weight class. As you may have noticed, something that uses an angry fusion cannon as an arm weapon is far from being a true Titan. The Ta'Unar Battlesuit sits around the Knight Titan classification.
KV-128 Stormsurge: After the Ta'Unar was deemed successful enough as a proof-of-concept, the Tau would used their newfound experience with Titan-building to create the Stormsurge, a physically lighter class of the newly-dubbed Ballistic Suits, the Tau name for Battlesuits that are too big to be even jokingly considered "suits of armor", and thus the official name for all Tau Titans. Fio'o Bork'an Ishu'ron, the mad Earth-caste genius behind the Stormsurge, designed the suit to be a really big XV88 Broadside, capable of taking on specifically Imperial heavy armor so long as it is not a Titan. It's main weapon is one of two variants of
Compensation Pulse Cannon: the Pulse Blastcannon is a goddamn tank-sized plasma shotgun (what for killing lots of little things dead), and the Pulse Driver Cannon which is probably ripped off of a direct-fire artillery emplacement (what for killing one big thing dead). The Stormsurge does not have arms, instead having two WALLS OF MISSLES which it uses to ruin your day, and the days of all of your infantry. It even has stabilizers, all the better to shoot you with, just like the Broadside does! The odd thing about the Stormsurge is that it probably wasn't made with the engineering experience gained from the Ta'Unar, because Fu'Rious Bork'ed Iso'hedron forgot to add a roof to his mech. Seriously, the thing is open-topped, showing off both of the two pilots to the sky. Maybe he thought that they would want to eyeball their shots from way up on top of their moving building of a suit. This compounds the matter of the Stormsurge's physical weakness, which is not as impressive as it's size would suggest. The Stormsurge, while as big as an Imperial Knight, can't really take one on at close range, so it is commonly deployed when the Tau need overwhelming firepower to face a larger force of conventional armor than they can handle with Hammerheads and Broadsides alone. Apparently the design philosophy of a walking tank didn't extend to its armor, still proving that even if the Tau can make a wall of guns, a wall of guns is not a structurally sound building practice.
Unfortunately fielding a titan in a tabletop game will cause lots of RAGE and presents itself as a huge fire-magnet, so every fucking thing on the other side will try to shoot at it. Luckily it's got shields, so it can soak up a lot of damage. Should said titan 'get killed' it may take out the whole field (if its a small one), and all those units around it, enhancing its awesomeness. Too bad that, since you are forced to buy one from Forge World, everyone who sees you with one will call you a rich noob who buys his way to victory.
(Anything larger than a Reaver has to be built from scratch. NOT ANY MORE!! For the price of a mere £1240 you can now buy a warlord titan model from Forge World, sure it's a rip-off but who cares. It's an awesome model. If you want to make an accurately scaled miniature of an Imperator, it's actually easier to stand on the table while wearing a sign saying "Imperator Titan"- it's that big.) AND, you have to play Apocalypse (or Escalation) which SUCKS Lord of Wars permitted in normal games from 7th edition. But who cares? YOU HAVE A GIANT RAPE MACHINE OF DEATH!!!!! (If you really want to field a Titan and you are smart enough to realise that apocalypse sucks, you could always organize a game where it's 1 Titan vs an equal points number of normal stuff. Would be a weird, probably unbalanced but fluffy and fun game (Having witnessed a single Ork Gargant with crew take out an entire Imperial Guard regiment on table top, it's not as unbalanced as you think). Or just a titan showdown, possibly 1v1.)
DISREGARD EVERYTHING. FORGE WORLD JUST RELEASED A FULL BUNDLE OF LIKE 5 TITANS FOR THE COST OF ONE WARLORD TITANS. RIP APOCALYPSE. ALSO TITAN LEGION GAMES ARE NOW POSSIBLE WITHOUT BEING A DRUG LORD.
Lukas the Trickster WAS able to troll Titans so hard that their owners and their dogs would rage quit instantly but, thanks to the 7th edition codex, he can't do that crazy shit anymore! His "The Last Laugh" special rule used to remove any model in base contact with him when he died, including Titans, but now functions only in a challenge, thankfully, meaning he can only threaten characters. His reign of terror is over (at least for Titans)!
All 4 main imperial titans in the Ultimate Apocalypse mod for Dawn of War. Tactical marines included for scale. The model for the Warlord is kinda shitty but the imperator is amazing.