Okay, after reading through some of the posts on this page, I'm completely confused. So the Night's King on the show isn't a corrupted Night's Watch commander who joined with the White Walkers and became their leader, he's just some First Man (presumably) that the Children turned into the first White Walker? Or is this just speculation?
Leaf didn't say it was the First Men per se, just "men", but whomever changed the wiki entries on Monday wrote:
""Before he became a White Walker, the Night King was a human 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 as Westeros was invaded by the
First Men, who were cutting their sacred trees and slaughtering the Children of the Forest.""
This is a reasonable assumption deduced because of the conflict between the Children and the First Men which took place in the Dawn Age. I checked the
timeline and this is the
only time these sides were fighting, so that eliminates the possibility of the Night King being a NW Commander from the Age of Heroes -at least in the TV series.
It also means (IF the show is following GRRM's timeline, that is) that the White Walkers were hanging out for two or three thousand years doing nothing but perfecting their ice sculpture skills
before getting around to invading Westeros during the Long Night.