Last night’s Game of Thrones might have been one of the season’s most heartbreaking episodes, but it was full of revelations about the series’ primary antagonists. The threat of the White Walkers — those icy, blue-eyed figures whose appearance is human-like — has rarely been higher than their capture of the the three-eyed raven’s cave in season six, episode five "The Door." For a few seasons we’ve known the White Walkers’ greatest weaknesses: weapons made of dragonglass and Valyrian steel. Wights, their pseudo-offspring of corpse soldiers, are also vulnerable to fire, and a battle with Jon Snow last season showed that it’s possible neither wights nor White Walkers are able to swim. But while time traveling last night, Bran Stark got a firsthand look at the White Walkers’ ruler, the horn-crowned Night’s King, and the mystical being’s beginnings. From his vision, Bran learned that the Children of the Forest — those leafy, child-like nymphs — created the White Walkers thousands of years ago when they battled the First Men, Westeros’ original human inhabitants. Last night Bran saw that the Night’s King was a fabled First Man until the Children of the Forest pinned him to their Weirwood tree, stabbed his heart with an obsidian dagger, and turned him into the first White Walker. Strategically this information doesn’t reveal a lot about how to defeat the White Walkers. But it does correct the conventional wisdom that they’re another foreign being that came from some distant northern land. White Walkers were created to fight the growing control of the First Men over Westeros, but they grew too powerful and began to battle the Children of the Forest too.