"Magic of some sort. And it seems like their touch against the dead will raise other dead to their cause." From what he's seen at least. Any dead that still had the ability to walk or fight had seemed to be able to raise and join them. Which was also concerning because any lost on their side might join the problem until they can finish it. "Though from what I've heard, take down the Night King permanently and the rest will fall. They're all tied to him and his magic."