I don't trust men who were cast away to the Citadel of Sorrow for treachery, even if they were able to survive the fires of damnation.
—Carden Mannux
The Citadel of Sorrow was a fortress on Ziost that served as a gateway to what the Sith referred to as the underworld. After the conclusion of the Great Territorial War, Darth Invidius was captured by Sith loyal to Darth Viea and they cast his soul away to the citadel for his treachery. Nearly one century later, Invidius was able to find a way to have someone in the realm of the living free him from the citadel, and the person who ended up doing so was Carden Mannux.