It is also hypothesized that the Drowned God is a thrall of The Great Other. Euron in the books, with his horn that can control dragons, is hypothesized to be the one to bring down the wall and let the white walkers pass. Like Cly said, the Night's King is right now just a sort of mythical figure that hasn't actually shown up yet. Things are going to be significantly different in the books when it comes to that, at least. ---
Moops? "I thought you were making up diseases? That's spontaneous dental hydroplosion."