Yu’vath

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.