Khangba Marwu was a vast prison complex located in the Himalaya Mountains, under the mountain known as Rakaposhi. It was maintained by the Legio Custodes. It was used to house enemies of the Imperium, including rival warlords defeated during the Unification Wars. Among its more famous prisoners were Uilleam the Red, the Ethnarch of the Caucasus Wastes, and many of Narthan Dume's lieutenants. After the beginning of the Horus Heresy members of the Traitor Legions were imprisoned in Khangba. Sections of the prison were designed specifically to house powerful psykers.1
1The Outcast Dead (Novel) by Graham McNeill, chapters 10-14 passim.