Game of Thrones has no shortage of mysterious, magical items. After all, this is a show that has dragons and resurrections as significant plot points! There's a particular magical item, though, that's been linked closely to a character's transformation without ever really explaining the details. Ever since Bran Stark has returned to Winterfell as the Three-Eyed Raven, he's been spending a lot of time in the godswood around a beautiful, eerie weirwood tree. Turns out, those trees have some serious history in the world of GOT.
According to Westerosi lore (which is more clearly laid out in the A Song of Ice and Fire novels), weirwood trees are deeply magical and ancient. Some of the trees bear carvings of faces, which were supposedly created by the Children of the Forest, who allied with the First Men thousands of years ago when the White Walkers first attempted to invade the continent. The specific tree where Bran has been spending time is the tree that guarded its cave even from the Night King himself. Although it's never made explicit, it appears that something about the weirwood tree has power against the White Walkers — which could definitely come in handy in the present day!
The show has never quite made it clear where the weirwood trees get their powers or why they're so magical in the first place. Unfortunately, the weirwood tree at Winterfell is even more special now for being one of the last of its kind. When the Andals invaded Westeros and conquered the First Men, they also brought their religion with them, worshiping the Seven instead of the Old Gods. In order to suppress the Old religion, they had many weirwood trees and godswoods cut down. Weirwood trees do still grow naturally, but only in the forests of the North (such as Winterfell) or beyond the Wall. While most of the focus so far has been on dragonglass as a weapon against the Walkers, it's possible the mysterious magic of the weirwood tree may come into play as well as the final battle looms.