In Destiny 2, the former emperor of the Cabal is Calus. He ruled the Cabal empire from the Leviathan, his enormous flagship, and became known for his lavish court, extravagant celebrations, and fascination with absolute loyalty. His reign ended when a coup led by Dominus Ghaul and other conspirators overthrew and exiled him.
After his exile, Calus continued to influence events through his Loyalists and invitations to Guardians aboard the Leviathan. He first appeared as the central antagonist of Destiny 2’s Leviathan raid, where Guardians fought a ceremonial version of him. Later stories revealed more about his ambitions, his strained relationship with his daughter Caiatl, and his desire to gain the Witness’s approval.
Ghaul was the military leader who replaced Calus, not the former emperor himself. Umun’arath was a Cabal commander, while Xivu Arath is a Hive god. Calus’s story eventually tied directly into the Darkness: he became a Disciple of the Witness before being defeated during the events surrounding The Witch Queen and Lightfall.