Sasuke will truly die. Same as how Madara died losing to Hashirama. Hashi said that he was certain that he killed Madara for the sake of the village.
I think that the mystery man will revive Sasuke. Same as he did to Madara back then. I think that the mystery man will reveal how Madara survived after losing to Hashi.
I still have no idea who is the mystery man but i think that he is from the uchiha clan.
A man can only turn evil when he experience bitterness in the battlefield. Maybe because of the uchiha losing the war to the senju clan. Or maybe he is one of the strongest ancestor of the uchiha that never moved on knowing that their clan lost to the senju.
So what do you think?