Being the greatest sorcerer in the world is easily undone by one chorae or hell even a sniper shot from an Inchoroi weapon of light. Kellhus couldn't know what he was up against. Makes sense that he'd make a pact with the Pit. No other way to condition the ground of the Ark without surveying it.

Ajokli's assistance was, imo, one hundred percent necessary. Kellhus just assumed ultimate his reason would allow him to maintain control. And then maybe he could betray Ajokli later and run some sort of additional scheme with heads and poles and whatnot.

But it failed.

Anyhow, that's just my two cents, as someone who hasn't gotten so deep into the crazy speculation game and hasn't caught up on all the deeper theories y'all are pondering yet.