Just finished reading this book for the second time. I'm a little confused as to who this guy is. Rand doesn't recognise him as one of the forsaken, but he could be in a disguise. He seems to wield the True Power, but the only one who's openly done that so far is Moridin and he doesn't seem like one to help Rand out. He could be another high-up darkfriend; keeping Rand alive could be part of their plans. His description (black coat, almost as tall as Rand) doesn't really match anyone that I can remember.
I haven't read any of the Brandon Sanderson ones yet so if the answer is revealed in them then I'm happy to just read on and find out (don't want the surprise spoilt!) If, however, I should have been able to figure out who he was then I'm stumped.