The Emperor's Horn
More than two thousands years ago, during the time of the Lord-Militant Angevin, the Expanse was home to an ancient and alien evil known as the Yu’vath. Twisted servants of the warp, they built an empire on a foundation of dark sorcery, soul slavery and daemon worship. For years the Crusade fought a bloody war against them, slowly pushing them back in the face of massed slave armies and vile warp-technology. Finally Saint Drusus himself struck the final blow and ended their domination over the Calyx Expanse. What was left of their worlds and artefacts were destroyed and their locations purged from Imperial records and over time their existence was slowly forgotten. Whispers persist however that they were never truly defeated and there are Yu’vath worlds still hidden deep within the darkness of the Expanse.