Honestly, I don't think he was much of a mystery. He is pretty straight Dunyain in PoN.
Pretty clear that in order to come before, you can't be preceded by anyone. In order to do that, he had to seize everything. Every person, ever philosophy, magic, every major player including the Consult, and even the only dunyain in play. He's a bit more mysterious now, but what remains, assuming he is still substantially Dunyain, he must seize the rest of humanity, the Nonmen, and the gods themselves.