RJ said that as of CoT(I think...WH at the earliest), we had not ever seen Demandred's alter ego. So he can't be Weirmon, since we met him in book 4. He is just a darkfriend.

My guess is Demandred is in Shara, and will begin aMoL sweeping across the waste with a huge force and lots of channies. Granted, I said that ToM would begin that way, so I could be wrong. But its the only place he could get a big enough force with enough channelers to challenge Rand on his own. And his primary reason for going to the shadow as to prove that he is a better general and channeler than LTT all on his own, so this is his last chance to prove that.

Edit: Oh, and we don't know that his light aura doesn't work on low level darkfriends...he makes one low level darkfriend tear his eyes out, and another commit suicide later just from indirectly seeing his aura. Granted, it is a further buffed up aura at that point, but still, it obviously applies to all darkfriends.