After a decade of having the rug pulled out from under them, fans of Game of Thrones are, on the whole, a paranoid lot. Between Ned's beheading, the Red Wedding, and Jon Snow being stabbed (and then resurrected) it's not surprising those who watch the show closely are primed to see conspiracies and clues everywhere they look. Some of these theories are downright insane. ("Varys is a merman," for instance.) Some are quite logical ("Winterfell may fall" is a totally valid theory.) But these connections between the Night King and the Targaryens are one of those theories that fall into a grey area, where it's hard to tell if the clues add up.
The biggest clue came last season when the Night King resurrected Viserion as an "Ice Dragon/Wight Dragon." Everything points to only Targaryens riding dragons. As the blood of Old Valyria, the dragons recognize Targaryen blood when they smell it. It's why they know Jon is the son of Rhaegar before either he or Daenerys realize it themselves.
So the sight of the Night King astride a dragon, and riding far more comfortably than either Daenerys or Jon did on their first trip out, seemed a pretty big clue. The argument against, naturally is that once death is involved, having Targaryen blood doesn't matter. The Night King resurrected and controls Viserion. His blood is not a factor.
But now there's a new theory tying in the Targaryens to the Night King, and it got a boost from the first episode in Season 8 when fans saw the scene with Beric and Tormund at Last Hearth. The Night King had left a message for them, nailed to the wall. Poor Ned Umber, in the center, dead, while arms were put up around him, forming a spiral.
It's not the first time fans have seen that symbol from the Night King. However, in previous iterations, it was formed from frozen body parts in the snow. Seeing it in fire format was different, and for more than a few fans, called to mind another symbol, one of "Fire and Blood."
It's a good observation, and one most would not necessarily notice, especially after living with the symbols of Stark, Lannister, and Targaryen over the last decade. After a while, something like that gets so commonplace, it fades into the background. And the bottom of the flaming spiral does call to mind the wings and tail of the three-headed beast.
But like with the Dragon riding theory, there's a big hole in this idea, namely that the Targaryens, and their symbol, did not originate on Westeros. As hardcore fans have consistently reminded everyone, there were no dragons the first time Westeros fought the Night King, nor were there Targaryens. In fact, the rise of Valyria wasn't for another two thousand years on Essos after the Night King fell. The drawings in the Dragonstone caves of this same spiral predate the Targaryens by centuries.
Chances are, this is merely a coincidence of design aesthetic by the show and not a clue that Daenerys is going to turn evil. But it's still a neat coincidence.