Symbols look familiar?
The same patterns the Children of the Forest drew inside the cave on Dragonstone have appeared on the show before: The White Walkers scattered victims’ bodies in one pattern in the season premiere, then they created the other pattern in horse carcasses at the Fist of the First Men in the third episode of Season 3; in a Bran flashback, we learned stones rest (or once rested) in a spiral pattern around the large weirwood tree where the Children of the Forest created the White Walkers.