The world of Thedias Prime and its surrounding planets were under a shadow. The shadow of the Great Devourer - summoned through the machinations of the work of a perfidious genestealer cult.
...and then an Imperial superweapon killed the entire hive fleet with a single cataclysmic psionic explosion, saving the sector and liberating the genestealer cult from hive mind control. However...the cult did not recruit entirely through mind control. Drawn together by shared disgust with Imperial...