Its all implied, not stated.
Something is occluding Kellhus' vision. Kellhus doesn't know what it is, or so he tells Proyas. It seems, post-TUC, that it makes sense for it to be Ajokli. From there, you go back and look at the series as a whole and think: if the end of Ajokli, how far back was he been involved? Since the beginning? Since the Curcumfixtion? Or maybe not until TAE and his travels to the outside.
Hard to say, but Ajokli seems to be the only culprit that fits, though pinning down how much and when is much harder.
See everyone points to the Circumfix, I don't. I think it's somewhere in that 20 year period we skip when he's plumbing the depths of hell. I've always thought the most crocadillian of Sons to be Ajokli. That's when I think discussions and contingencies took place.
The Circumfix doesn't fit well with me at all. I think that is Bakker backtracking. Which is why I don't take his extratextual comments too seriously. Of course we're deprived his POV for 2 books, I don't know how much we would glean. Bakker was only willing to give so much on Kellhus, the Golden Room was his "g-string" moment.
There are two places think Kellhus could be and one I have a very neat theory on. One, obviously, the decapitants. But, the other struck me as me and H was talking about Kellhus mastering the Daimos to such an extent that he could be in the Outside, just in his own niche.
Hear me out. The Ekkinhu (Sp?, sorry I haven't downloaded the kindle book. New phone.). Maybe all those mysterious deaths that (whoever found them, come forth so I can acknowledge you) were to create the thing. I think there is a passage in TUC about one could see souls and such. Maybe when he died his soul shot straight for it. A means of creating his own niche in the Outside, as you would most definitely need souls to do so. See, I don't think Ajokli is the one talking to Kellhus in his visions, I think it was Kellhus. Even though he said the voice was crazy and he ceased to listen. He didn't, obviously and they needed crazy to do what they was doing. I think he's created his own heaven. Because in the dreams it sure as heck doesn't seem a hell. It is serene, it is Dunyain.