War of Dark Revelations
After barely repelling Hive Fleet Gorgon, the Tau Empire gained a new fear of the Tyranids swarm. Now under threat from a tendril of Hive Fleet Kraken, the Tau desperately sought an advantage anywhere they could. It was at this desperate time when a Dark Eldar Haemonculus named Urien Rakarth contacted the Tau High Command and made a proposal: he and his Haemonculi Coven, The Prophets of Flesh, would join the war against the Tyranids in return for what he called a 'cultural exchange'. Looking for allies anywhere they could, the Tau eagerly agreed. Initially the new alliance seemed to be working out marvelously: Rakarth's soldiers, grotesque but mighty scythe-limbed bare-chested warriors, managed to inflict terrible losses on the Tyranids on the Tau Sept of Vigos. These fiendish creations seemed nearly impervious to any kind of pain and despite taking heavy losses the silent warriors were decisive in turning back the Tyranid advance. These abominations were soon aided by legions of Haemonculi, Talos Pain Engines, and Cronos Parasite Engines.
Though the Haemonculi's warriors were stomach-churning, they had turned the tide on a dozen fronts and the Tau were highly impressed. But soon Urien Rakarth appeared before the Tau once more demanding his price: seventy-seven Tau from each Caste were to be given over to him, including seven Ethereals. Though the Tau deemed the Ethereal price unacceptable, they were prepared to pay almost any other price for the Greater Good of their empire and agreed to Rakarths terms, delivering the Tau tithe by Manta through a crackling Dark Eldar portal in the sky.
The next phase of the war against Hive Fleet Kraken was a focused counter-attack. Once more The Prophets of Flesh of Urien Rakarth agreed to aid the Tau. Battling the Tyranids alongside Tau Fire Warriors, the Dark Eldar deployed a new weapon: long-limbed Grotesques that moaned in despair as they hacked through the Tyranid horde. After six days of carnage the Tau and insidious creations of Rakarth were victorious. However a terrible suspicion began to dawn. Vid-captures from Tau Drones showed that the Grotesques of Rakarth were of a blue-grey coloration all too familiar to the Tau. Mere seconds after the Tau realized to their horror what had become of their "cultural exchange", Rakarth demanded via video screen that the Tau hand over the seven Ethereals or 7,077 other Tau in their place.
Realizing the kind of monster Rakarth was, the outraged Tau refused the deal and became determined to punish the Haemonculus Lord for his crimes. Mobilizing their fleet reserves from the nearby planet Rubikon, the Tau made course for Rakarth's fleet in orbit. However the Haemonculi fleet turned out to be nothing but mirages and empty space. Confused, the Tau then received a desperate message from Commander O'Shaev detailing that Rubikon was now under attack by Rakarth's forces. Now undefended, Rubikon was the subject of a massive bloodbath that saw even Supreme Overlord of Commorragh Asdrubael Vect himself and his Kabal of the Black Heart involved in the fray. The Prophets of Flesh deployed thousands of Wracks, Grotesques, and Pain Engines which overwhelmed the spirited Tau defense. When Tau reinforcements finally arrived, they found Rubikon to be a barren world, its populace stolen away to the horrors of Commorragh.