Here’s what we do know:
• He was once a human who was turned into a White Walker by the Children of the Forest thousands of years ago, to become a weapon against humans.
• He has the ability to turn others into White Walkers and wights.
• At some point, the Children of the Forest lost control of the White Walkers.
• The Children of the Forest and humans made peace in order to fight the White Walkers together. They didn’t kill the Night King or the White Walkers but something happened that caused them to go into hibernation/hiding for thousands of years.
• He’s really good at javelin.