The Night King was once a mortal man, one of the First Men who waged a lengthy war with the Children of the Forest. As the Children were losing the war, a small group of greenseers captured the unfortunate man and bound him to a heart tree to be used for a terrible purpose. Leaf, who was among the group, pressed a cursed dragonglass dagger into the captive's chest, causing his eyes to turn blue and turning him into the first of the White Walkers. Thousands of years later, Leaf explains to Bran Stark that her people created the White Walkers to defend themselves when Westeros was invaded by the First Men, who were cutting their sacred trees down and slaughtering the Children of the Forest.