The Damocles Crusade (also known as the Damocles Gulf Crusade) was a major campaign against the Tau Empire by the Imperium of Man. Since it was first added to the lore it's gone through several additions, expansions and most importantly massive fucking retcons.
The First Draft
During what the Tau called the Second Sphere Expansion, the Tau's colonization efforts brought them across the Damocles Gulf and on the other side they found human worlds. These frontier planets were easy to subvert; they had little attachment to the Imperium thanks to being cut off from it for some time, thanks to both Eldar corsairs and warp storms. There were still some loyalist factions among these planets but they were in the minority.
When the warp storms receded the Imperium decided to restore its grip in the region, and it was thanks to system defence ships that they noticed the Tau. A report came in of a Tau ship that didn't stop at the designated checkpoints, and we should get into some backstory on that. At the time the writers wanted to the Imperium to be more practical than stupid, so the Imperium would not intentionally shoot every alien race on sight (they still aimed their guns at them) because they knew they couldn't afford to fight an active war against literally everyone else in the setting. In regions where there were either sanctioned species or areas that required brief cohabitation with xenos species there would be designated checkpoints set up to guard Imperial space, done so that they could inspect xenos ships to make sure they weren't going to negatively affect Imperial colonies/worlds.
Once the Tau ran the border they were immediately attacked and the Tau were shocked to learn conflict could escalate that quickly. They tried to make a speedy retreat, but the ship ended up being destroyed by the Imperial Navy and the matter briefly considered closed. It was reopened again when an investigation was launched into what they were after Adeptus Mechanicus Genetors learned that this advanced alien race matched those of a primitive alien race found six thousand years ago.
Not one to miss an opportunity, Rogue Traders and merchants who didn't particularly care who they dealt with went to explore the eastern fringes and they started 'less' violent meetings with the Tau (how much less is up for debate). Luckily they were so far away from the main body of the Imperium that word of what they were doing was slow to get to the Administratum, allowing members of the Water Caste to establish trade agreements for goods and technology (this was also a time where the Tau weren't convinced the only way to get resources was to kill and enslave those who had them). This would later become the main cause of the war, as a large amount of alien goods and artefacts began to appear in nearby systems and an investigation would later reveal the Tau's allies within dissident Imperial factions.
When word finally did get back to the Administratum they completely lost their shit (to put it lightly) then told the Ecclesiarchy who also completely lost their shit. A 'suitable' response began to be readied, deployed a mere century later with the goal of purging the Tau. The response consisted of a dozen capital ships, five provisional companies of Space Marines made up of contingents from a dozen Chapters, and nineteen regiments of Imperial Guard.
To say the Tau were ready for the response is like saying that a watermelon was ready for a sledgehammer. The Imperium blitzed into Tau space; their first targets were worlds in which human colonists committed the crime of co-existing with the Tau, but since this was the practical version of the Imperium, instead of blowing up the planets they punished the humans who dealt with the Tau, purged the aliens and left everyone else alone with a threat. After this they engaged the Tau properly in the uninhabited Hydass system.
The Imperials engaged with seven Tau warships, and it was their first real insight into what the aliens were capable of. The Tau had fantastic long-range capabilities and escorts that were great at protecting their ships from Imperial bombers, but a Space Marine strike cruiser ended up turning the fight in the Imperial's favour when it managed to get inside the Tau's formation and let off everything it had. This turned out to be a great idea since the Tau warships lacked broadside firepower; afterwards most of the Tau warships managed to retreat from the fight with the only confirmed kill being one unlucky bastard (probably the one the Marines opened up on) which was destroyed before it could be fully evacuated.
The remnants of the Tau fleet regrouped in the Sy'l'kell system, which the Imperials wanted to destroy for being too annoying to pronounce. It was home to a fertile agri-world consisting of a population estimated to be 7 million Tau, defended by an orbital station along with their aforementioned ships. Once again the Imperial strategy was to take a page from the Orks' playbook: het in close and krump 'em! It worked; the Iron Hands (the ones in charge of the planetary assault) boarded the station and overran it, burning all traces of the Tau personnel out with flamers before repurposing it as the command center for the ground campaign (not heresy, remember this is back when the Imperium had Reasonable Marines). The Tau fleet fled after the station was boarded as they knew how well they Tau performed in close combat, but in doing so they managed to avoid taking any significant damage.
If the Tau found themselves struggling to fight in space, the opposite ended up being true in the ground conflict. When Imperials came down they found the Tau quick to adapt to their tactics, responding to aggression with an extremely well-equipped mechanized infantry formation backed up by armoured columns. This was also the first time the Imperium witnessed a soon-to-be classic sight: a Hammerhead turning a Leman Russ into Swiss cheese. The Imperium also found out the hard way that any unit left unsupported would soon be in a world of trouble, discovered when crisis suits nearly wiped out the entirety of an Imperial Guard company by swooping in without notice (the massacre only being prevented by both Stormtrooper and Scythes of the Emperor reinforcements). Still, the Tau couldn't hope to match the Imperium's numbers and once the Fire caste fell, the rest of the world went with it. The Tau evacuated the most qualified workers and dismantled any advanced technology they didn't want the Imperials to nab (presumably if they'd met the AdMech before this they wouldn't have bothered) and the Imperium cleansed the planet.
A short break was called as the Imperium took to going over their collective plans again for taking Dal'yth, but in the meantime an expeditionary force was sent over to deal with Tau in the Viss'el system. Unlike the Tau in the Sy'l'kell system, those who lived in Viss'el inhabited an ice-covered water planet of no strategic value. Not seeing any purpose in landing on a planet that they weren't going to use in the foreseeable future, the Imperials performed an orbital bombardment, melting the ice around the main colonies and drowning the poor bastards.
The attempt at taking the Dal'yth system is what finally ground the crusade to a halt. Actually getting there took five months of journey (one can only imagine what it would be like for the Tau going the opposite direction) and even when they arrived they did so piecemeal. The arrival itself wasn't any better than the travel time; when the Imperials arrived they found Pra'yen was protected by an orbital station similar to that guarding Sy'l'kell but significantly more dangerous, being fitted with an array of heavy railguns. Its first volley crippled an Imperial cruiser, and eleven Tau ships emerged behind the planet to support it. The Imperium broke formation in order to engage them and kept to the usual strategy of 'get close and broadside them.' Interestingly the Imperium used their escorts on the orbital station instead of devoting their ships to their usual support role against an enemy fleet, which turned out to be the correct thing to do as while they took heavy losses, they were more effective against it then they had been against the Warships (where they'd performed terribly thanks to the Tau's escorts).
Just when it looked like the Imperials were about to win, the Tau unveiled their trump card: having planned for the Imperials to come in close they decided to rely on their Kroot allies, and ordered in a Kroot Warsphere since it was more capable than the Tau at that distance. Unfortunately for them, the Imperium was quick to realize that the Kroot's guns were laughably short-ranged and the sphere itself, while huge, was also slow. Before it could reach the fleet in full (who were beginning to scatter, which would've put the ball in the Tau's court) it was engaged by one of the flagships, the Blade of Woe. The Admiral in command accelerated his ship to its maximum speed and tore across the Warsphere, and its ensuing salvo ripped the giant ship apart. The end result was a Pyrrhic victory: the Tau fleet, the Warsphere and station were destroyed at a cost of four capital ships and fourteen escorts. At this point those in charge began to respect the Tau's ability to fight and for a while they considered turning back, only to find the lure of a major Tau world too tempting.
Dal'yth Prime was next on the Imperium's 'worlds to fuck up' list, and at first they approached the planet cautiously as it was protected by THREE orbital stations. Fortunately for the Imperials the Tau had made the RTS mistake of only investing in defences along your border wall and had only one huge armada, so none of the stations were anywhere close to as dangerous as the one at Pra'yen, and after approaching them carefully the Imperium found they could be destroyed at their leisure. Expecting another Tau fleet at any moment (one which later turned out to never be coming as the Tau really did put all their eggs in one basket) the Imperium immediately began the ground assault, only to find that it was even worse than the last time they engaged the Tau on a planet's surface.
To begin with the drop was contested by large numbers of Tau flyers, and while the drop was eventually successful it bought the Tau the time they needed to set up. The Imperials headed for the nearest city, Titans and armoured formations clearing the way and ploughing through the streets, but they were beset upon by enemy missiles fired from hidden positions well beyond their own range, taking heavy losses in the process. Units had to be detached to drive back Tau spotters, which they couldn't do because they were beset by Stealth Suits, which they couldn't fight because that required the guns the missiles were taking down, which they couldn't deal with because the Tau were driving the Imperials fucking crazy. Gradually the Tau broke up the Imperial advance until it consisted of three spearheads instead of one large force, and they appropriately picked out which units would fight against each spearhead. The one made up of Titans was countered by Tau Manta Missile Destroyers. The second spearhead was predominantly made up of veteran light infantry who liked to cover camp since the Tau's markerlights were too busy being used on tanks to take away the infantry's cover saves. They were beset upon by Kroot who managed to do the whole 'urban combat' fighting a lot better than the regular Tau did. The third spearhead consisted of Space Marines, a mix of chapters from the Iron Hands, the Ultramarines and the Scythes of the Emperor. They were countered by Crisis Suits, and together the three counters stalled the Imperial advance once they'd left the city, causing the front to stabilize twenty miles north of the initial city's center, along the line of a broad river.
Combat on Dal'yth took months, mostly because the Imperium found it hard to engage with the Tau in a number of conflicts. The Tau possessed superior night-fighting capabilities which they exploited every chance they got, opening up fire when it was pitch black only to withdraw at dawn before the Imperials could fully get a grasp on what they were fighting. The worry of a new fleet continued to gnaw at them as the current Imperial fleet would not be able to take on the same sized force as last time, and while plentiful reinforcements had been promised at the outset of the crusade none had arrived. One General concluded that as they stood, they did not have the resources for the task at hand, while an Inquisitor was thinking now would be a good time to evacuate the planet and then blow it up (not really a surprise, all things considered) as he reasoned the Tau were too dangerous to be allowed to exist. The only reason Dal'yth didn't become another molten hellhole is because a number of the other commanders found the Tau to be honourable in a fight and the thought of having to resort to exterminatus instead of cleansing the planet properly left a bad taste in their mouths (though aside from arguments about the best way to purge the Tau they didn't actually try to stop the Inquisitor from ordering it, and the crusade ended before he did).
While the other commanders debated about which xenocide is the best xenocide, a General down on the ground decided to get shit done. He formed the titans up with the Imperial Guard regiments and attempted to force the river line down its right flank, trapping the Tau against the sea. He did this after pounding other sectors the Tau could've retreated to with artillery and ordering Imperial Navy fighters to begin offensives against Tau airfields which, while costly, did prevent the Tau flyers from taking to the front. This proved to be highly successful at first, changing the battle line in the Imperium's favour and gaining the ground troops easy access to a bridge that would allow their forces to cross the river and get to the city on the other side without much fear of retaliation, until the Manta missile destroyers showed up again. After they reappeared a running battle was fought through Dal'yth's suburbs, ending with the Tau falling back and infesting the now fallen city with Kroot, though the Imperium wouldn't end up fighting them because they'd managed to seize a starport and the General decided it was time to get the hell out of dodge. He began evacuating the troops from the planet, and surprisingly enough the Tau decided not to do anything when the Imperials were evacuating, reasoning that there was no benefit in impeding their enemy's retreat (especially when that might piss them off enough to come back for another go) and both sides held parleys under the flags of truce. There was quite a bit of resistance to this evacuation by both the Space Marines and the Inquisition, but the General found himself to be vindicated when a call came in from Inquisitor Kryptman: Hive Fleet Behemoth had just arrived and they were needed elsewhere.
Later the terms of the truce between the Tau and the Imperium would be hammered out, but in both cases it was a facade of peace. After the Imperials left the Tau went on to retake their lost worlds, assimilating some groups of human deserters/renegades in the process. In return the Imperials preyed upon them like a malicious scientist observing how their test subjects squirm, abducting them for whatever experiments they wanted to do while sowing mistrust and doubts about the Farsight Enclaves (even managing to get the Water caste to talk about the 'Tau mercenaries' as they called them). The end result of the conflict is the Imperials learned how the Tau fight and gained respect for them as warriors (the only thing they'd end up respecting them for), while the Tau found their optimism about how they could easily take the galaxy had been checked. In the end the Tau would have likely made further gains into Imperial space, but by then Hive Fleet Kraken showed up on their doorstep, at which point they went from checked optimism to THIS GALAXY IS FUCKED UP.
The Second Draft
The first re-write/retcon of what happened isn't quite as well told as it makes the Tau come off as being stupid rather than naive. For the sake of brevity the changes will be the main focus instead of just reposting the entire story.
In 6th edition the story was changed so that instead of encountering Imperials on Imperial worlds, the Tau came across free captains, pirates and lost human colonies that had regressed so far back they'd forgotten their origins and sometimes even their birth worlds. These initial contacts ranged from Rogue Trader to Tyranid levels of hostility, and somehow the Water caste was able to figure out that these were just remnants or willing outcastes of an incomprehensibly large galactic empire. The codex makes it clear that the Tau believed the Imperium would crush them like blueberries if they pissed them off, but despite this the Fire Caste wanted to pick a fight with them anyway instead of doing fucking anything else.
Thankfully Aun'Va (before he was retconned into being evil incarnate) was in charge and decided to try a different approach: integrate Water caste members into human worlds who were still fully loyal to the Imperium, insinuate them into the courts of Planetary Governors and slowly break them off from the Imperium's control. Through bad writing the Tau are not terminated on sight (remember, these are worlds completely loyal to the Imperium, not the dissident worlds that were previously in the story) and their plan was successful, managing to get several Imperial worlds to even choose the Tau over each other as trading partners (ignoring the logistics of why this wasn't a thing before). Soon Tau technology flowed through the human markets yet somehow nobody on this formerly loyal planet noticed anything strange going on. Eventually the Water caste convinced the Planetary Governors to side with them over the Imperium and they announced their independence, finally drawing the Imperium's attention.
All of the Imperium's earlier battles (including the battle of Pra'yen) become a footnote while the Inquisition is more brutal on the worlds that chose to rebel, with the battle of Dal'yth also being sizably cut down. An odd note that appears in this rendition is that the Tau are mentioned as possibly being able to encircle or blockade the Imperials evacuating the planet, only choosing not to thanks to the Ethereals who presumably realized that doing so would be literally impossible, seeing as how the Tau had no air support or navy, and encircling them with land forces at a starport would do nothing.
It should also be noted that at this point the Tyranid hive fleet entering Tau space is entirely forgotten about, presumably because the cliffhanger the last story left on was too interesting to continue. This also marked the beginning of the Tyranids attacking the Tau while the Tau are in a bad spot, only to be retconned out the next time this same story comes up (so far it's happened at least three separate times in three different stories).
Damocles, who the campaign and region was named after, was a man in Ancient Sicillia who was a stern fan of the local tyrant called Dionysios and considered him to be very fortunate. Dionysios wondered why Damocles liked his job and his kingdom so much, and went to teach him a lesson. He invited Damocles for a party at his residence, which Damocles of course was super-excited about. There was only one thing that Dionysios hadn't told Damocles, and that was that, while eating, Damocles would be laying under a hanging sword, attached to the ceiling by a single hair from a horse' mane.
After a very strained and unhappy dinner, Dionysios explained the act to Damocles: he wanted Damocles to feel like he does as a tyrant. A tyrant always has a sword over him, that can fall down and strike him down at any moment. He is never at peace and can never relax, but can't do anything about it either as that would just result in killing innocents and doing heinous, but ultimately useless acts. Damocles was scarred for life, but had learnt a lesson. Never again did he praise Dionysios.
What has that to do with the Damocles Crusade? Well, the Imperium did attack the Tau Empire, a faction notorious for being unassailable by your usual means, and ultimately ended up being humbled by their futile act, while the Tau Empire only grew stronger from the experience...
My God, they've got no clue.
250 years later, after dealing with Ork and Tyranid invasions into their territory while fighting small local wars against the Imperium, the Tau launched their own full scale invasion of the Damocles sector.
It started with a series of lightning strikes against the Zeist sector to which the Imperium responded by sending almost all available Guard forces from nearby sectors as well as a chapter's worth of Space Marines, mainly from Ultramarines successor chapters led by Cato Sicarius. At first it appeared to be a great Imperial victory as the space marines were able to drive the Tau raiding parties out of the sector. However, this was just a distraction to bleed the Damocles sector of reinforcements as the stationed Tau forces (which Sicarius mistakenly estimated as a million) masquerading as the entire Third Sphere army was really a token force while the main force stood primed to flood the Damocles Sector. Way to go being Shadowsun’s unwitting puppet, golden boy! To add insult to injury, the Tau learned that little trick from the Ultramarines when Ageman and his First Company pulled something very similar on a much smaller scale about 50 years before during a smaller engagement.
The Tau then came in force, guns blazing, with a goal to take over the entire Damocles sector, conquering poorly garrisoned planets in the dozens while Imperial forces were scrambling to retreat and regroup. The first real battle happened on the hive-world of Agrellan, a major logistics node instrumental to taking control over the Gilded Worlds, which were a tightly packed group of mineral-rich planets in the center of the sector.
Despite an overwhelming victory for the Air caste in orbit, the stubborn Imperium still managed deploy reinforcements of White Scars, Imperial Knights of house Terryn and Catachan Jungle Fighters to aid local defenders. Initial phases of the planetary assault didn't go well for the defenders, as Shassera laid siege on the Prime hive-city in the blind zones of its artillery and forced the Imperials to sally out, only for them to fall into a textbook Kauyon trap. Following this Knighs and Catachans wisely decided to refrain from further counterattacks but not the Scars, who mounted a second sally-out with a goal to hunt down Shadowsun and cut off her head. Against an opponent infamous for her cunning bait-and-ambush tactics it went about as smoothly as you would expect. Marine strike force lost all of its aircraft (roughly 30% of manpower) and only managed to break encirclement and retreat back to the Hive due to
gue'la mind-science a psychic storm summoned by Scar Stormseer and unexpected help from the Raven Guard who manged to outsneak and outambush Tau ambushers before they locked down the White Scar hunt completely. This set an overall tone for all White Scar engagements for the rest of crusade.
At this point both forces received new orders from their high command - the Tau to take Agrellan quickly with a swift blow rather than a series of patient and cost-effective but time-consuming engagements, and the Imperials to evacuate the world and join combined sector forces gathered to stop Tau invasion. While the Tau are generally uncomfortable with sieges and city-fighting, preferring to lure their enemies to more favorable positions, there were simply no alternatives to taking down Agrellan if they wanted to conquer the Damocles Gulf in time, so Shadowsun took a page out of Farsight's book and planned and executed a MULTIPLE, SIMULTANEOUS, AND DEVASTATING DEFENSIVE DEEP STRIKES on Imperial defenses using all the cheese she could get, namely spamming the new Riptide suits humans weren't familiar with. Even though they were outnumbered six to one (before you call bullshit this is including PDF that made the bulk of imperial forces), the Tau scored a decisive victory and took down
Agrellan Hive Prime* in a single day. Thanks in no small part to Riptides serving as Distraction Carnifexes, Shadowsun's infiltration force killed Imperial Command HQ with perfectly placed shots, resulting in guardsmen running around like headless chickens; on top of that one of the new Tau experimental weapons accidentally blew up a fucking moon. Even while White Scars, Raven Guard and some Knights of House Terryn put some spot-on resistance, the rape train gained too much momentum and it had no brakes – the only thing the marines and knights manged to do was to pierce the encirclement to allow a few survivors to evacuate and barely secure LZs to let that evacuation happen, and it cost them. Agrellan was taken over by the Tau Empire and renamed to Mu'Gulath Bay - The Gate of New Hope - meant to become a new sept world and the base of operations for entire Third Sphere Army.
*Later they realized that taking a whole fucking planet "in a single day" bit was really fucking stupid (and physically impossible) and retconned it; Agrellan was home to seven hive cities, and each one needed to be besieged individually. This took several weeks all told.
In an uncharacteristic display of hot-headedness (likely coaxed by daemonic whispers of the Agrellan wastes), Shadowsun lead a pursuit of the retreating imperials to a Voltoris - a knight world of house Terryn and the last piece of meaningful Imperial resistance in the sector. She intended to stomp them for good instead of evading them like she had done the entire campaign), but it was a trap; the pursuit force got decimated and Kor'sarro Khan wounds her though she managed to escape with her head still on her shoulders. Continuing the trend for Tau possessing Astartes grade plot armour this minor engagement was the only true defeat the Tau had through the whole campaign, though it caused Shadowsun to regain her cool and return to her regular cold-hearted calculative bitch behavior, mainly because Aun'Va decided to show up in person at this point.
Following their advance towards the Gilded Worlds, the Tau attacked the fortress world of Perfectia, but their old friends, the Raven Guard, White Scars and the Knights of House Terryn were already there, with the Raven Guard bringing almost their entire chapter. The war turned into a a contest of cunning with Raven Guard captains and Fire Caste commanders trying to outsmart each other with limited success for both sides; while the Raven Guard was familiar with that kind of war, the White Scars ended up completely out of their field of expertise and in a hilarious chain of fake retreats and ambushes, Shasera out-tricked Kor'Sarro so hard Tzeentch himself took a moment to admire the amount of Just As Planned involved. The resulting clusterfuck ended with a massive battle where Shadowsun proved herself a superior trap mistress and even pulled an Alpharius gambit on the Raven Guard chapter master, blowing him to bits with her new Ghostkeel suit while giving her own XV-22 to a sitting duck volunteer. White Scars retaliated with
FOUL SORCERY psychic storms conjured by their Stormseer conclave (turns out the librarian survived the melta shot from the previous book), but once again it was, while effective, not effective enough to do anything but buy more time. When the war turned into a meat grinder despite Shasera's best effort to end it quickly and decisively, the deployment of the new Stormsurge suits proved instrumental to fighting off imperial Knights, and when surviving Knights begun their retreat and evacuation, the rest of the Imperials broke and retreated too. While the marines continued to resist and Shrike even lead a successful counter-attack to retake Severax's gene-seed, the war was pretty much over. It wasn't as curb-stompy as the Mu'Gulath victory, but despite suffering heavy losses (like it was an IA book) the Tau conquered Perfectia and renamed it Vas'Talos ("Boundless Scope"; note "Talos" was a Greek giant treasure guardian, kinda like this sept guarding the advance on the Guilded worlds).
While Tau propaganda hailed it as a brilliant victory, it took too much time and too many bodies, and the Imperium launched a counter-invasion on
Mu'Gulath Agrellan, and this time - following their dogma of "throwing bodies at it until it's fixed" - arrived with a shit-ton of guardsmen and Leman Russes (but they kinda forget to send anything else), who are only good at "proper", frontal fights, being easy prey for the Kauyon tactics the Tau had been using the entire campaign. Imperium, being Imperium, merely orders them to march forward nonetheless, expect to be ambushed and hope for the best.
This in turn allowed the badly outnumbered and besieged Tau to inflict much more casualties a force their size had any right to, but since the Imperium always loses IQ points when they face the Tau, they just threw MOAR men AND tanks at them, turning the second Agrellan campaign into a shameless tank porn fest, mainly because most of the battle zones were almost featureless irradiated deserts with a few ruins and dunes here and there (Agrellan's a re-settled virus-bombed planet after all). Pask and Longstrike covered the hulls of their vehicles with kill marks, and rows of Leman Russes got outranged by rows of Hammerheads (even though they have the same effective range) while silly infantrymen were mowed to red or blue paste in between them and air support was mostly unavailable because of the pollution/sand-storms of the planet (which were actually controlled by the Tau). The Imperium actually manages to tank through their initial losses, cut off Tau retreat, and start pounding them into the dust through sheer belligerence and pinpoint shooting from Knight-Commander Pask. The battle continues to escalate as Imperial forces press onward to stop the Tau from retreating and regrouping to their ambush tactics forcing Shadowsun out of her Kauyon element, and requiring her to commit more and more of her limited reserves into a standup fight that the Tau forces couldn't possibly win. Complicating matters further, the danger to Aun'va's life clouded Shadowsun's judgement and prevented her from committing all of her forces at a critical moment, which allowed Imperial forces to deflect a key counterattack and begin turning what had grown into one of the largest battles of the campaign into a rout in the Imperium's favor.
Then Farsight drops into the fray at the critical moment, Manta striking out of orbit from ships that seemed to appear quite literally out of thin air (presumably he's been spying on the Tau Empire and intercepted the distress call). Despite having declared Farsight a outcast and a rebel, the Tau are emboldened and manage to rally to his Enclaves, slowing the Imperial invasion down to a crawl with a violent stalemate over the world. Despite their victories, because the Tau are still severely outnumbered they cannot hold their ground and end up falling back to the final shielded fortress, 'shielded against orbital attacks (because that makes sense on a frontier world that had very recently faced war), built over the ruins of the former capital hive.
Rather than do the sensible thing that would have ended the war then and there, like have Terminators teleport next to the shield generator to destroy it, or have Assault Marines drop in to destroy it (like they were doing earlier in the exact same fucking book), the Imperium decides to engage in a proper static fight (their specialty), in this case an offensive siege. However, Farsight, master of Mont'Ka, in a book called Mont'Ka performs a, you guessed it... Kauyon while Shadowsun... also performs a Kauyon that the book calls a Mont'ka. This trap was meant to trick the Space Marines into attacking what looked to be a bunch of Tau training; despite being nowhere near the important battle, offering no strategic value to the Imperium, and being in an area that could've been leveled with ordnance or orbital strikes, they take the bait and attack, having learned no lessons from their earlier engagements and forgetting to bring vital equipment like scanners so that the trap would work. They end up caught in it and do not contribute to the final battle in any way. Shrike and two full companies aren't fooled by the trap though, and manage to massacre the Tau in a melee ambush, though 200 (give or take a few) Space Marines running rampant behind Tau lines amounts to absolutely nothing and the book very quickly forgets that they're even around. Luckily for the Tau, the author liked them and so the Imperium was more than happy to send all of their Imperial Guard walking (what, you expected them to have chimeras?) in easy-to-pick-off formations directly towards the guns that were specialized in picking them off (sending vehicles right in front of the stormsurge suits for example) rather than going around to a part of the city that wasn't as heavily defended and attacking there (in case you're curious, yes the there was more than one way into the city and the Tau should not have had the numbers to cover all fronts), and since the Mechanicus forgot that they're supposed to hate the Tau, they don't attack either even though attacking through ruined cities is something the Skitarii excel at. Since the Imperial Guard cannot achieve anything without Space Marine assistance, and the stormsurge suits are invincible whenever they want to be, the Tau actually manage to utterly defeat the Imperials on a frontal fight across the entire city even without assistance or guidance from Farsight or Shadowsun for a good portion of the fight (despite saying the two were critical beforehand), and despite the fact that they did not have the numbers needed to hold the Imperium back in earlier static fights. As you might have gathered at this point its going to become pretty much standard that any Imperial commander facing Tau have to turn into drooling morons and fail at even the one thing the Imperium are supposed to be good at.
While this was going on, the Imperium decides to employ the Officio Assassinorum Execution Force, using the final battle as a cover, on four of the top Tau Leaders: Shadowsun, Farsight, Farsight again (because they knew who the Deus Ex Machina was), and Aun'Va. While the first three end in some degree of failure or another due to plot armour (the Tau's perfect reactions), the Callidus manages to kill one random Ethereal (this has no impact even though it's supposed to traumatize all the Tau who hear of it), and the Culexus sent to kill Aun'Va succeeds by using his pariah abilities to literally hitch a ride into Aun'va's complex, murder his guards, and then brutally dispatching the Ethereal Supreme, leading to the actual death of a named character with a model for the first time in... fuck, FOREVER. In order to keep everyone calm, they replace the xeno-pope with a perfect hologram they apparently just had lying around as they decid to fall back. You'd think they themselves would despair over losing at least two Ethereals, but luckily Shadowsun's mind is plot armoured too, and Farsight no longer cares. And to put in a final insult, the
Adeptus Mechanicus Admiral Hawke decide to drop the Exterminatus on Agrellan and the Adeptus Mechanicus did something to erect a massive firewall which made the Damocles Gulf impassable to the Tau (presumably passable by Warp-travel and the like), cutting any Tau colonies off from supplies from the empire proper. Luckily the main Tau building was outfitted with plot armour shield generators that can magically withstand a planet-killing weapon and the firestorm simply kills anyone outside of it (including millions of Tau and imperial leftovers) without cooking the people inside (think bread in an oven). Agrellan is lost, the Tau military forces evacuate safely (though how they leave the planet and how Farsight returns to his Enclaves is left up to your imagination) and decide to learn how to build firewall immune ships. This causes massive casualties in the process, but failure is the mother of innovation, and EVERYTHING causes massive casualties in 40k anyway. Especially when your "invention" is just a very poor copy of Imperial and Kroot designs without Gellar Fields.
In the end, Shrike now has to take on bigger roles, Kor'sarro's still salty about getting owned, and Shadowsun and Farsight decide to agree to disagree, if only because they don't have the taupower to waste on killing each other. And speaking of that red bastard, he has to leave because his Enclaves are being eaten by Tyranids.
- Aun'Va has been replaced by some holos and some other ethereal may be doing the speech writing job.
- Farsight enclaves are getting NOM NOMned by the tyranids... AGAIN.
- The Imperium manages to lose yet another huge battleforce (mostly off-screen). Do notice the Raven Guard get Boreale'd (also off-screen), and while GW will talk about some rubbish like honour in sacrifice and keeping the spirit in dark times this pretty much confirms they are becoming just another punching bag fallen to the level of the Astra Militarum *BLA-* BURST CANNON FIRE! PEWPEWPEW! Yeah, pretty much what happened. (As if they weren't before).
- The battles could be described as "BUT THEN RESERVES ARRIVED". Everyone tries to out-Alpharius each other; too bad the imperials had to work with whatever was at hand and Tau brought a full crusade with them. And, because this is a Tau book, when the Imperial reinforcements do arrive the Tau get yet more unexpected reinforcements in the form of Farsight popping out of the
bluered and the writers decide to make the victory look good by blowing the brains out of the Imperial Commanders so they fail to understand basic military concepts or even manage to send the right equipment to the right sectors.
- The battles could be described as "BUT THEN RESERVES ARRIVED". Everyone tries to out-Alpharius each other; too bad the imperials had to work with whatever was at hand and Tau brought a full crusade with them. And, because this is a Tau book, when the Imperial reinforcements do arrive the Tau get yet more unexpected reinforcements in the form of Farsight popping out of the
- The Mechanicus decided to show how it's done and set fire to the nebulae, as it seems they are the only human force which took everlasting measures in the wake of the battlegroup's failure. If you were expecting much AdMech involvement, sorry; they just snatch xeno-tech (Heresy!) and return to orbit, completely forgetting that they're supposed to hate the Tau to the point that they want them all dead. Not content with merely abandoning their IG allies, they keep to watching the many ways they die (mainly to see if there was anything else to steal). There was, so they send an entire Skitarii battleforce to try and abduct some stormsurge suits, which catches the Tau in an ambush... only to cut to Pask doing what he always does, the Skitarii forgotten for the rest of the book. If anyone even remotely won, it was them.
Are you starting to see a pattern here? Most of these campaign books seem to be ending with Exterminatus thrown in in such an unorthodox and complicated way we are starting to wonder where the traditional methods went.
IRL Tau gundams sell extremely well, expect next-gen battlesuits to conquer the entire Eastern Fringe FOR THE GREATER anime show (No, seriously, their holos are used as propaganda among the Tau kids)! And then got NOMNOMed by the Tyranids to boost their sales.