Who in the last 3000 years has done more to brings things to their current state than Ishamael? Who has done more in the Dark One's name than good ole Ishy? You got it, no one. Granted he may have been a bit whacky there in the end, and may have even thought that he himself was the dark one, but the results are what matter. I personally don't see the Dark One being picky over his minions harboring an insane thought or two, or even uttering the occasional blasphemous phrase he doesn't even mind when they kill each other. What he is picky about is results. He wants the job done and he wants it done right, and so far as I have seen, Ishy has been the only one capable of doing that. So who better for the DO to reward with a bit of elevation above his cohorts than Ishy? No one. That is why I believe that the DO has rewarded Ishamael not only with reincarnation, but with reincarnation as The Hand Of The Dark and the numero uno servant of Big Ole Nasty.