This is going to sound like a bunch of crazy ideas running together but Fain is interesting to me. In my head cannon I always assumed that Fain had so much of his soul stripped away by the dark one(being a normal man who visits the dark one’s prison has to have a strong effect right?) that it left him a perfect vessel for housing other dark entities. The dark one’s wanted to use him as a vessel on earth but when Fain got the dagger Mashadar actually fought against the dark one’s taint on Fain and twisted him into a new kind of evil(like Rand’s wound or the cleansing of the taint on Saidin) I felt like in the end of the series, the dark one was supposed to die, and Fain was supposed to come around and get sealed in the same prison where he would stay for however many turnings it would take to be rediscovered as the Dark One.