The Night King's origin goes back to a time when the Children of the Forest were in a tough spot against the First Men. They took a human captive and used their magic with dragonglass to turn him into the Night King. This was a huge mistake in a way. After becoming the Night King, he started to amass an army of the dead. His army consisted of wights, which were re - animated corpses. He had an ice - cold stare and a sense of purpose that was both terrifying and mysterious. He seemed to want to cover the world in ice and snow, and his power was such that he could raise the dead just by his will. His back story is a key part of the lore in 'Game of Thrones', and it added a lot of mystery and horror to the series.
The Night King was created by the Children of the Forest. Long ago, they were at war with the First Men. In an attempt to gain an edge in the war, the Children of the Forest used dragonglass to transform a man into the first White Walker, which eventually became the Night King. He then started to raise an army of the dead, and his goal seemed to be to bring an eternal winter to the world of Westeros.