"A weapon the size of a Titan. A near-perfect expression of the Omnissiah's grandeur and, of course, his puissance. Armies are thrown to the wind by its glorious song, and mountains themselves skitter aside at its demand. Nothing stands before a sacred Machine Spirit so mighty without its consent."
- – Tech-Priest Garrulon Vyme
In the rare event that even the Titan Legions just can't get the job done, the Imperium of Man brings in the specialized war-machines known as the Ordinati. These are basically unintelligent (so far as we know) Bolos built with Imperial technology. These things are basically the angry black matriarchs of the AdMech's arsenals.
Unlike the standardized Titans, each Ordinatus is custom-built by the Adeptus Mechanicus for a specific objective and named after the world it was constructed on. A special branch of the Admech called the Centurio Ordinatus is responsible for their creation, use, and maintenance, similar to the Collegia Titanica. The Ordinati themselves vary widely in armaments, size, shape, and role - some are armed with guns that would make an Imperator Battle Titan jealous, others are designed to carry entire battalions of troops, but all are more than capable of winning a war on their own. Most of them are only used for one battle and then get mothballed.
Using each one for only a single fight seems kind of wasteful given how stupidly powerful they're supposed to be, but you must first realize how the Mechanicum sees them. To the AdMech, innovation is one of the most dangerous things in the galaxy - if they invent a new Ordinatus it is only out of extremely dire need, and even then only the Centurio Ordinatus Magi are entrusted to do so. After it fulfills its purpose, the new Ordinatus should by all means be left alone unless it needs to perform its specialized role again. After all, it's a new technology not based at all on any pre-existing STC, and as such using it without a very, very good reason is seen as sacrilege if not outright tech-heresy, and if you push the wrong button you might vaporize somebody's favorite mountain by accident.
In light of that, actually deploying one at all is a very rare event compared to that of their Titan counterparts. Most of them were either destroyed, lost, damaged beyond repair, or just plain forgotten during the Horus Heresy. They're typically deployed separately from any Titan Legions or Skitarii, and are used only when any other options have failed. One was constructed in the middle of the First battle for Ullanor in the war of the beast, and fired PLASMA DRIVES (famous for powering BATTLE BARGES on their own). This was so effective, it could one shot a column of Ork Gargants. Unfortunately it met a sticky end when the Beast's PERSONAL TEMPLE-GARGANT fired all its weaponry (and there's a lot) at it, and still only just managed to cripple it. WOW.
Ordinatus engines are absolutely massive land crawlers that can run over and squash Baneblades, and are protected by a specialized energy field called a Dispersion Field on par with a Titan's void shields. Unlike the Titans, Ordinati are usually not meant to directly assault enemy positions, but tend to be better suited as really big back-line artillery. They are (very relatively) squishy for their size and firepower - and fairly indiscriminate at unleashing said firepower - so they need support armies to keep enemies at distance as they blow everything within several hundred miles of them to pieces.
Speaking about firepower... While Titans and huge ass super heavy tanks tend to have multiple guns on them, Ordinatus engines always mount only one ridiculously OP weapon system which can be as large as an actual Titan. Most (if not all) of those systems were once specifically designed to solve one particularly difficult problem. Let me reiterate this: The Mechanicum, the guys who see innovation as one of the most perilous things in the galaxy, actually invented these things. Not just "in the distant past before much knowledge had being lost" but right the fuck NOW - some of the famous Ordinatus weapons were invented in M41 to fight off Waaagh! Ghazghkull. It's no surprise to anyone that the cogboys consider them the most sacred "relics" that they possess.
The "small" Ordinati, which are still each the size of three Leman Russ tanks lined up end-to-end, are typically armed with weaponry normally seen on Titans. They're easily recognized in silhouette by the two pairs of tracks they possess: two at the front and two at the rear. All of the big, "regular" Ordinati have three pairs of treads. Granted, each of those treads could wholly crush an Ordinatus Minoris, but that is besides the point.
They're also the only Ordinati which have rules for them available and they can be taken in both 30k and
40k (40k rules awaiting the release of Fires of Cyraxus) Admech lists. Both of the available Ordinati Minoris are Super-Heavy Vehicles with AV14/13/13, 14 Hull points, Blessed Autosimalcrum, Armored Ceramite, Anbaric Claws, an Ordinatus Dispersion Field (an improved Flare Shield that's most effective early in the game: initially it grants a -3 S to all shooting attacks and rolls on the D chart, which goes down by one until the third turn, where it remains at a -1 penalty to all incoming shooting attacks and strength D hits), three turret-mounted Volkite Culverins, and a built in 6++ save. The catch is that they also have their own Catastrophic Damage table in which the closest range is always Strength D no matter what gets rolled, so if it ever does go down it's guaranteed to fuck over anyone nearby.
Currently, two Ordinatus Minoris variants have their own rules:
- Ordinatus Sagittar: Did you ever say to yourself "Hey, I wish I could mount a Titan-grade Volcano Cannon on something other than a Shadowsword"? Well, you're in luck- for "only" 700 points, the Ordinatus Sagittar can fire off a blast of Strength D Apocalyptic Blast goodness from 180' away, allowing you to snipe enemy vehicles and everything vaguely close to them from the other side of the table. The Machine Destroyer rule on it is a little overkill, but very nice against Titans.
- Ordinatus Ulator: 1075 points nets you the Ordinatus Mars' little brother, armed with its own Sonic Destructor. At first glance, its rules are nasty as hell- 72' range, AP1, Armorbane, Instant Death, Pinning, and Ignores Cover in one package. However, the meat of its effectiveness comes from its "Ulator Sonic Wave" rule. Instead of measuring the shot normally, you place a Massive Blast marker so it touches the Ordinatus' hull, and move it forward in any direction from its firing arc (so long as the first model hit by it is an enemy; mind you, friendly units can be hit afterwards) until it either reaches the max range or it leaves the table. Everything that the template passes over even partially is automatically hit, with squads taking a number of hits equal to the number of models that the template covered as it moved. Yes, this includes fliers too. From there, its strength changes depending on what got hit- it acts like S5 on infantry/bikes/beasts/jetbikes/cavalry, S8 on MCs and non-tank vehicles, S10 on tanks, and the almighty D to super-heavies, buildings, and GCs. Between its ability to tear apart an entire army in one shot regardless of what's being fielded and the fact that it literally cannot miss, it says a lot that this is still one of the "small" Ordinati. In addition, it sounds something like this.
During the Schism of Mars, one traitor Magos holed himself up in a mountain citadel, sending his armies to harass Loyalist factories. Four companies of Imperial Fists led by Sigismund were sent to stop him, but discovered that the Magos possessed a peculiar xeno artifact that caused any ship that got too close to it to be blown up or thrown into the Warp. A ground attack was decided to be a more viable approach...if it weren't for the half-mile thick walls made of the toughest materials known to the Imperium.
The Mechanicum responded by creating the Ordinatus Mars, armed with a Sonic Disruptor that caused the citadel to shake itself apart with the force of its sound waves. As the walls of the fortress crumpled the Loyalist Tech-Guard was able to storm the citadel and execute the traitor Magos with no difficulty.
After Ghazghkull Mag Uruk Thraka fled Armageddon, Yarrick managed to track him down to the planet Golgotha. Some time after his capture by the greenskins and later escape, he returned with a punitive force of Titans and Skitarii. The sheer size of the Ork WAAAGH! made it impossible for them to gain an advantage, warranting the creation of an Ordinatus made specifically to deal with overwhelmingly massive hordes.
The Golgotha's Hellfire Missiles were perfect for the job, and routed the Orks within a week. Since then it's been deployed in any battle where the enemies of the Imperium outnumber even the Imperial Guard.
The Ordinatus Armageddon was designed during the First War for Armageddon to defend against a particularly dangerous horde of Daemon Engines. The plasma-based cannon gives it the ability to melt Titans in one shot and disintegrate entire tank battalions with a salvo. Supposedly, it was inspired by an old Squat anti-Titan vehicle. The book Helsreach speculates that the Ordinatus Armageddon's main gun was basically a Nova Cannon (a weapon typically used only on Imperial Navy warships) modified to use an auto-loading system instead of a crew of thousands of slave workers.
It is the only 'large' Ordinatus that can be properly estimated based on size. While not official, it can assumed that its maximum size puts it in the same weight category as Imperial Destroyers or Frigates. We got this size by estimating its Nova Cannon with that of Imperial Battleships and scaling it with the Armageddon. Of course do note that this is just taking into the presumption that the Nova Cannon on the Armageddon is the same size as those found on Battleships and that changes in its size can fluctuate. This is unlikely to be true because a titan is a hell of a lot smaller than a ship and requiring a "salvo" to destroy a tank battalion implies (relatively) rapid fire capability so the shots must be far smaller than those of a true nova cannon. A nova cannon in the Navy can wipe out an entire squadron of escorts with one shot and those presumably are flying thousands of kilometers apart from each other. Besides, a novel is not necessarily canon (pun?) and a plasma-based weapon is probably exactly that, a plasma weapon. So, we have no real knowledge of how big it is or what it shoots except that it is likely no smaller than a Capital Imperialis.
Essentially the Hades Breaching Drill's big brother, the Priam was designed during the Siege of Priam to bypass the defenses of an occupied city by drilling directly through a planet's crust, traveling through the mantle underneath, and then reemerging behind enemy lines. It was successful at deploying four companies of Imperial Guard to break the aforementioned siege without incinerating them in the process, but was rendered inoperable afterwards due to its shielding partially failing during the attack. Since most Ordinati are only intended for one battle anyway, this wasn't an issue. More importantly, if four companies of IG could break the seige from inside, just who the hell were those guys? Spartans?
The Gehenna was armed with an Apocalypse Cannon strong enough to shoot straight through a planet's crust hard enough to trigger magma flows. It was fought over by the Imperium and Chaos during the 13th Black Crusade.
Ordinatus Volcanis Ultor
A rare example of a Traitor Ordinatus, made by the Dark Mechanicum of Volcanis Ultor to wipe out an advancing force of Grey Knights. Its exact armament was never described, but judging by the description of it being able to fire pinpoint salvos of multiple warheads it may have been capable of firing missiles or shells of some kind. However, it was never actually used- the wreckage of a Grey Knight Strike Cruiser that had just been destroyed by the traitors' warships crashed on top of the war machine, obliterating its cannon barrel and ammunition loading systems by accident.
A destroyed Capitol Imperialis command vehicle and a plasma battery taken from a downed Ark Mechanicus, the Ordinatus Ullanor lived a brief life during the battle against WAAAAGH! Beast during the events of the Beast Arises series. It showed up, killed a few Gargants, then got nuked by the beast's mountain sized Gargant in one shot.
The Ordinatus Mars. Suck on it, Noise Marines.