The Somnus Citadel was the headquarters of the Sisters of Silence during the Great Crusade. Based on Luna, it contained launch bays for Black Ships and was an impressive fortress armed with automated gun-drones. In addition, parts of the citadel could be separated and lowered underground in case of attack.
During the early stages of the Horus Heresy, the survivors of the Eisenstein were interred within the Somnus Citadel on the orders of Rogal Dorn. Battle in the citadel took place when the terminally ill Eisenstein survivor Solun Decius gave himself over to Nurgle. Mutating into a Warp abomination, Decius slew numerous Sisters of Silence and loyalist Death Guard until he was killed by Nathaniel Garro. After the incident, the Somnus Citadel was used as the base of operations for Malcador's newly formed Knights-Errant.