spoiler !!! do not cross

Before he became a white walker, the night king was a first man that was captured by the children of the forest, leaf among them. leaf pressed a dragonglass dagger into his chest, causing his eyes to turn blue and turning him into the first of the white walkers. thousands of years later, Leaf tells 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 and slaughtering the Children of the Forest

anyway he became winner of olympic games (throwing of spear)