The Night King was a good villain, though my least favorite out of the main bad guys in the series. In the books, this incarnation doesn’t seem to exist, but in the show he’s certainly the mastermind and king of the dead. We eventually find out that the White Walkers were Craster’s sons turned into evil creatures of darkness.
And that the Night King was once a normal man. Turned into a super-weapon by the Children of the Forest to fight the First Men. The Children of the Forest must have realized their mistake and ironically allied with the First Men to defeat him. Though he must have retreated to safety. Not coming out of his hiding until after a huge wall was built to keep him out.
He tricks Bran into leaving a magic mark on him during a vision and he’s able to get into the Three Eyed Raven’s lair and kill almost everyone, while wights killed Summer and Hodor.
He soon becomes a big enemy to Lord Commander Jon Snow. It doesn’t take long until Jon Snow realizes who this being is, and how terrible he can be.
As he can easily raise wights from the recently dead. Not a guy you’d want to fight in a “fair” battle.
While Jon was trying to capture a wight to prove to the rest of the Seven Kingdoms, the Night King and his army make a mess of it. He kills Dany’s least powerful dragon but all the heroes except Benjen Stark survive.
Which is ironic considering he uses Viserion to burn down the wall of ice and magic with magical blue fire. Jon would have been smarter to have just let the Night King keep Beyond the Wall as his own kingdom now that nobody lived there.
During the final battle at the Long Night, Drogon hits the Night King at point blank range with a impressive bout of fire. Since fire never seemed to affect the Night King before (in minor moments) I wasn’t shocked when he came out without a scratch…burn.
He fights his way easily to Brandon Stark and slays Theon Greyjoy. He does seem to wait a dumb amount of time to kill Bran Stark. He could have easily had the wights do it anyway.
But because of his waiting, Arya Stark comes out of nowhere and luckily manages to destroy the Night King with a Valyrian steel dagger to his ice heart. With his demise, all of White Walkers and wights are quickly destroyed.