Catalexis

This artwork was commissioned by me and drawn by TinnBSCTS:
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The Cacodominus was an extremely powerful xenos cyborg behind the Catalexis Heresy of M34.
A psyker of Alpha-level or higher, the Cacodominus was able to exert complete control over an area of 1,300 planetary systems. It was eventually slain by the Black Templars, but this would result in the infamous event known as "The Howling." The xenos' psychic death scream echoed and amplified through the Warp, burning out the minds of a billion astropaths and even distorting the signal of the Astronomican. This would result in millions of ships being lost in the Warp and entire Sub-Sectors descending into barbarism and anarchy.

The Black Templars have never bore Librarians since this time (theorized from the Cacodominus destroying all their Librarians or crippling their geneseed) but they still carry the Skull of the Cacodominus as a trophy, the relic still echoing with the xenos' monstrous psychic screams.