Hostiles - The Heretic Astartes
The Heretic Astartes abroad the Blackstone Fortress served under Abaddon the Despoiler in several campaigns, most notably in naval battles over Cadia in the 13th Black Crusade. As Cadia was destroyed, the explosion threw their Battlebarge into the Warp, where it emerged deep in Segmentum Pacificus inside the Blackstone Fortress. The Ship melded with the Fortress, stranding those on board inside. Since then, the Heretic Astartes seek to taint the Fortress and turn it against the Imperium of Mankind.