In season seven, Viserion is tragically shot out of the sky by the Night King during a battle beyond the Wall. After sinking to his death in an icy lake, the White Walkers haul the dragon's body out and the Night King uses his magic to revive him; next thing we know, he's riding Viserion and destroying the Wall. Could he actually be the third rider in the prophecy, or is the fact Viserion is no longer (technically) among the living enough to disqualify them both from the race?
Since Viserion is simply an undead version of a dragon, like a wight is an undead, zombified version of a human, he's able to be ridden by humans and undead creatures alike (aka the Night King). But if the Night King is killed, will Viserion die as well? And is there a chance Viserion could ever be swayed to be under Daenerys's control once more?