The Catelexis Heresy was an Imperial conflict that broke out in 401.M34, after the xenos cyborg known as the Cacodominus used his formidable psychic presence to control the populace of 1,300 Imperial planetary systems.
The zealous Black Templars Space Marine Chapter called a Crusade against the vile aliens and ended the Catelexis Heresy when they executed the Cacodominus. Alas, this outcome had unexpected consequences that would soon prove dire for the Imperium of Man, resulting in the infamous event known as "The Howling."
The Cacodominus' death scream echoed though the Warp and was amplified by it, burning out the minds of billions of Astropaths and even distorting the signal of the Astronomican. Millions upon millions of Imperial starships were lost in the resulting upheaval as entire Sub-sectors slid into barbarism without the dictats of the Adeptus Terra to guide them.
- Warhammer 40,000 Rulebook (5th Edition), pg. 124