So one part I never could figure was who the NK was, what's he looking for, and how his story would be closed in S8. Now I think I know.
1- I said before; the Children of the Forest created the NK in their fight against the first men, like one would build robots to serve and protect them. They eventually came to a truce with the first men, who converted to the Children's "religion".
2- The NK is controlled by a greenseer, using the Horn of Winter, just like how Dragon Binder works: if you blow it, you die, but you warg into his head. There must always be one controlling his mind, although it's possible that originally this was not needed and a greenseer could just take control of his mind. It was probably always a Stark making this sacrifice.
3- The CotF, the first men, the giants, all worked together to build the Wall. The wall would protect all of those north of it. The Night Watch and its castles were created on the south side to protect the wall against the south. That's why in the books there are even passages leading from the castles to the other side of the wall: the other side was the Night Watch's backyard, not what they guarded against. The Starks were the lords of the north, protecting the Night Watch and everything north.
4- Eventually, a Brandon was supposed to be the new one in line to sound the horn and die to take control of the NK's mind, like other Starks before him, but he refused. He destroyed the Horn of Winter at the Fist of the First Men, and he came to be known as Brandon the Breaker. The alliance was broken. So the situation turned around, the NW now had to protect the south from the north, and the Starks' history from before breaking the alliance was forgotten or erased.
5- Now, Bran is expected by the ones still allied to the CotF (Benjen, the Reeds, the Three Eyed Crow, the Children of the Forest), to retake control of the NK's mind. How? Not clear, maybe he just can, maybe this kind of greenseer hasn't existed in very long if not forever, and it's why the horn was needed in the first place. The NK would want Bran dead to avoid being retaken control of.
Chances are when you blow the horn of winter you get frozen. So the Starks in the crypts, the really old ones, would be frozen, and they are those who successively sacrificed themselves to warg into the NK.
Brandon the Breaker, having refused, would have went to attack and kill the Night King at the Night Fort, so the NK was routed north, but the freefolk didn't want the NK around so Joramun teamed up with Brandon and they pushed the NK further north. Together they broke the Horn of Winter. One piece was burried at the Fist, the other kept by the freefolk, and it the books that is the horn Mance had, part of it, but Melisandre burned it.
The prince who was promised is someone the allies of the Children had promised would sacrifice himself to warg into the NK every time the NK was about to regain his senses.
The reason the NK started sacrificing children was because since Brandon the Breaker refused to warg into him, whoever had before him was starting to vanish from the NK's mind.
