In Destiny 2, the Cabal leader who attacked the Last City at the start of the game was Dominus Ghaul. He commanded the Red Legion, an elite Cabal force that launched the surprise invasion opening the Red War campaign.
Ghaul had overthrown Emperor Calus and claimed leadership of the Cabal Empire. His objective was not simply to conquer Earth: he wanted to seize the Traveler and prove that the Light could be taken and controlled. His forces attached a massive cage to the Traveler, suppressing the Light and cutting Guardians off from their powers.
The assault destroyed the original Tower and forced humanity into retreat. Ghaul also captured the Speaker, hoping to learn more about the Traveler’s choosing of Guardians. The Guardian eventually helped organize a counteroffensive, returned to the City, and defeated Ghaul. Calus is a different Cabal ruler, while Umun'arath and Xivu Arath belong to separate storylines.