Well, when Kellhus does his, "The logos has no beginning or end, The logos has no beginning or, The logos has no beginning......" little drill with the Monk, he is taking control of that inner voice. As has Koringhus and Serwe, I assume. Its basically just your conscious, as we all know can play dirty tricks on you - hence, The Darkness That Comes Before. Now, bare with me here because my understanding of the psychological and metaphysical is minute. But, "A head on a pole behind you"= The Darkness That Comes Before. So that inner voice is basically a "real thing" that precedes you if you allow it, it is the darkness that comes before. So, Onkis being a diety of a head on a pole and she is also an aspect of the God, the darkness that comes before.
Now, if Locke is right, and the Nonmen created the Gods and knew the Aspects and such. Their name for Yatwer is the fertility principle(?), I'd love to know what Onkis's is.