The Koronus Expanse is rife with unnatural drifts of dead stars, each a dim ember, the remnant of a mighty cataclysm wherein a star convulsed in death throes, casting forth a shell of burning outer matter into the voids. The dark outer reaches of the Rifts of Hecaton were long ago sculpted by the violent deaths of dozens, perhaps even hundreds, of stars. A great evil indeed must have moved across the Expanse in a past age, a dark power that murdered the very stars, cluster by cluster. Dead xenos worlds orbit these stellar remnants, blasted molten in ancient death throes and then frozen beneath the wan light of a star-ember. Their ruins are cyclopean, their under-crust warrens labyrinthine, and their dire symbols, where not worn to dust by the passage of aeons, warn of xenos sorcery and ancient doom; they speak of the Yu’vath.
The Yu’vath were a blight upon the Calyx Expanse in the era of Lord-Militant Angevin, prior to the creation of the Calixis Sector in the fourth century of the 39th millennium. The warp-worshipping Yu’vath enslaved worlds though vile xenos sorcery, expending the lives and souls of corrupted human slaves to build such horrors upon the Calyx Hell Worlds that chroniclers of the Angevin Crusade forbore to record them. The warp-ridden Yu’vath and their tormented slave armies bled the Crusade’s forces for decades, including battling the titans of Legio Venator, before their ultimate extinction and the destruction of their homeworld at the hands of General (later Saint) Drusus and the Adeptus Astartes. It was rumored that the Yu'Vath were the ones to kill Drusus before his resurrection, but none can be certain. Even now, chronicles of the Angevin Crusade’s victories over the Yu’vath are restricted works, and few savants know what came before the blessed establishment of the Calixis Sector.
Remnants of the Yu'Vath are rare indeed, and contain frightening technology that drives the minds of whose who come across them mad. Such massive vaults include Designate-228-18 in the Calixis Sector. Remnants of their society were first found by the Disciples of Thule.