Season 8 finished its first outing on a bloody note with a mysterious message from the Night King on the Game of Thrones premiere. At House Umber in Last Hearth (the closest city to The Wall), Tormund, Beric, and their group found young Ned Umber tacked to the wall surrounded by a spiral made of human hands and arms. Ned had been turned undead by the Night King's advancing army, so Beric had to set the boy and the spiral on fire to kill the wight version of Ned for good.
As they watched the flaming spiral, Beric said it's a message from the Night King, and the spiral imagery on Game of Thrones goes way, way back. As Polygon noted, the White Walker symbol can be seen in several scenes, usually after the White Walkers attacked. They often took parts of their victims and make them into these spirals. But the spiral was also present in Bran's vision of when the first white walker was created by the Children of the Forest. Therefore, the White Walkers' connection to this imagery began when they began. Jon Snow also found similar spirals carved into the rocks beneath Dragonstone — Daenerys' homeland.
It's significant that the spirals can be tied to Daenerys, because some people also think the spiral is connected to the Targaryen sigil. That banner features a red three-headed dragon with its wings, heads, and tail circled around the body in a spiral formation not unlike the flaming one fans saw at the end of the episode.
The White Walkers are clearly into waging this war for more than just death if they're creating tableaus with their victims. At its most basic level, the message is a warning — the army is advancing and taking victims north of The Wall. At its most in-depth, the meaning could be that the White Walkers could be waging this war to help Daenerys take over the seven kingdoms. She's long claimed it was her birthright, and perhaps the White Walkers agree. It would also explain why the Night King re-animated her dragon after its death. Perhaps he intends to return Viserion to his rightful owner. There are theories that the Night King is a Targaryen, after all.
But it's also possible that the spiral imagery has just been co-opted by the White Walkers, but that it originally meant something else. As The Daily Dot noted, when Jon found the symbols at Dragonstone, he told Daenerys about how the Children of the Forest created White Walkers to fight the humans, but ended up having to team up with the humans to force the White Walkers away.
"They fought together against their common enemy," Jon said. "Despite their differences, despite their suspicions. Together. We need to do the same if we’re gonna survive. Because the enemy is real. It’s always been real."
To Jon, the spiral image seems to represent the coming together of the seven kingdoms to defeat the White Walkers. It's worth noting that, in most iterations of the spiral, there are eight arms — one for each of the kingdoms and an extra one perhaps to represent the king/queen? To truly defeat the threat of winter, all the kingdoms and Cersei need to band together. (Currently, Cersei seems to be ignoring this threat, so she needs to get on board.)
According to Time, co-showrunner David Benioff commented on the cave drawings in one post-episode discussion where he said that the White Walkers stole the imagery from the Children of the Forest.
"It's supposed to be something that's very evocative of the thousands of years that have passed since these caves were first explored and the paintings were first made. And also obviously something that’s quite relevant to the current storyline because it’s about how these two disparate peoples united to defeat the common enemy," he said. "One of the things we learn from these cave paintings is that the White Walkers didn’t come up with those images, they derived them from their creators, the Children of the Forest. These are patterns that have mystical significance for the Children of the Forest. We're not sure exactly what they signify, but spiral patterns are important in a lot of different cultures in our world, and it makes sense that they would be in this world as well."
The White Walkers' co-opted message may be a threat, but the original message seems to be one of uniting for the common good. Hopefully, that's the message the humans choose to listen to as this war wages on.