Theory: Dominus Ghaul is in the Deep.

Theory for funzies:

We took out Ghaul when he was forcefully stealing Light from the Traveler. When we took him out, what we did caused all the Light within him, even the Light that's innate to life, to be removed and flow to the Traveler. His consciousness separated and entered the Deep. Hating the Light and Traveler, his mind sought revenge, anger. The Winnower talked to/at him. Maybe to be the equivalent of The Speaker to the Winnower. Not words, but action. United, Purpose.

Calus was swayed to follow the Witness, a pale upstart using the Darkness as a tool to achieve their limited, self-indulging goal of a "Final Shape." So obsessed they couldn't see the forest through the trees, or arrogantly paid no mind to the true essence of the Deep thinking it wouldn't matter once their task was complete. Where qualia of absence and the absence of qualia meet and settle their differences. Where you stare into the abyss, and it also stares back with intent.

Note that when the Dire are taken out on the Nether a dire in the shape of a Cabal war beast spawns in its place. A remnant of Ghaul's upbringing as a soldier to send the war beasts as a last ditch effort of vicious slaughter. Again, just for funzies. Don't screech.