Warning: plenty of spoilers for Game of Thrones below!
We are knee-deep in trying to figure out how the final season of Game of Thrones is going to pan out, and a good chunk of our ideas revolve around finally figuring out the mystery behind the Night King. The ultimate villain of the series—— yes, he's even more of a villain than Cersei Lannister — is a total enigma, save for the few nuggets of clues we've gathered on what he actually wants. We don't even know what he's planning to do if he actually wins and destroys all Westerosi kind.
What we do know is that the Night King is one bad dude. He has no qualms massacring massive swarms of people, and nothing seems to stop him, not even the threat of Daenerys's three dragons when they meet in season seven. In fact, the Night King seems more than ready to handle them all, as he takes Viserion down with one javelin throw. Disregarding how freakishly prepared he seems to be in any situation, this threw fans for a total loop, especially because we soon see the Night King turning the dead dragon into another one of his zombie slaves. Next thing we know, Viserion is breathing blue fire and taking down the Wall, all with the Night King perched happily on his back.
Whoa doggy, we thought only Targaryens could ride dragons! Technically, that's true, as House Targaryen is the only major Valyrian house remaining in Westeros (on the show, there's a few more around in the books), so they would be the only ones capable of riding dragons. But that doesn't mean that the Night King is a Targaryen. In fact, the Night King is known to be one of the First Men, the original human inhabitants of Westeros before the Andals invaded. He is more likely to be an ancestor of House Stark than Targaryen. So, how can he ride a dragon? Magic.
The thing about the Night King riding Viserion is that he can only do so after the dragon is dead. Which means that whatever old Valyrian magic that was once within Viserion is gone. He's no longer connected to his mother and no longer fit for a human rider. The Night King uses his power to raise Viserion from the dead, the same way he's raised wights from the dead. That means Viserion is filled with the Night King's magic, which is why his fire is blue and why the Night King is able to use him as a mount. Viserion is now under the Night King's control and will obey his commands — there's no secret lineage at work here, just really messed up magic.
The real question is, how will the living defeat an undead dragon? The automatic assumption is that they'll kill Viserion with fire, just like with other wights. So Daenerys or Jon will have to defeat the Night King's mount with their respective mounts, Drogon or Rhaegal. We can't imagine it'll be easy for Daenerys to watch her child go down again — it might even lead to her own death. We'll just have to see what happens when the great battle of Winterfell begins!