In the world of Game of Thrones, the Night King and the White Walkers were created by the Children of the Forest as protection from the First Men, who sought to kill the Children and cut down the sacred forests. The Night King quickly turned on the Children of the Forest, leading to the first Long Night and invasion of Westeros. After warring for  a generation, the Night King and his White Walker army were pushed back to the North, and a wall was constructed to invent future invasion. 8,000 years later, the Night King has returned, with his army and wielding his mighty ice sword, and he is poised to invade Westeros again.