The Ebony Citadel of Mallyx is a heavily fortified building found in the center of the Domain of Anguish. It is the home of Mallyx the Unyielding and the only way to breach its gate is to defeat the four Lords of Anguish.
Breaching the citadel and defeating Mallyx is the topic of the quest Mallyx the Unyielding and The Ebony Citadel of Mallyx mission.