I think it is pretty much confirmed that the Night's King was never killed just banished. In this last season, we see a white walker with a white crown, dressed all in black, with 12 other black robed white walkers behind him, before he touches craster's baby son.
This is the nights king of legend, I am almost certain of it. I know there are a lot of characters who are black and white in terms of morality, but I think its safe to say that after thousands of years of exile, and the atrocities he's committed when he was still human, the Night's King is pretty bad news.
I wonder though, just about his life. Was the beautiful white woman he fell in love with an actual female white walker? or are they genderless, and one just took the shape of a woman to seduce him?
What sort of atrocities did he commit? He sacrificed people I know, and it says he enslaved some. Enslaved them to do what? I am also surprised the King in the North of that time cared enough to stop him as well as the Wildling King, unless of course his power was growing too great and he was becoming too monstrous..
Any thoughts on the once and future Nights' King?