In Season 8, Bran reveals that the Night King wants to destroy him because he holds all of the history of man.
"He wants to erase this world and I am its memory," he explains. As the Three-Eyed Raven, Bran has the ability of greensight, which means he can see events in the past, present, and future through visions. Killing Bran would essentially erase all memory of mankind. Samwell Tarly explains it pretty well in the scene.
So Bran and the Night King are connected to each other by the magic of the Children of the Forest, as well as to the weirwood trees that contain both magic and memories.
Bran Became The Night King By Accident
People are always warning Bran about warging for too long, so the idea here is that he tries to warg into the captive First Man to help him escape, only his consciousness becomes trapped in his body and Bran has essentially been the Night King this entire time.
Why did Bran become a king? Bran became King because he was the only surviving legal heir to the Iron Throne. As the only living son of Ned Stark, he is the rightful heir to the Northern lands, and his siblings, Jon and Sansa, both had renounced their claims to the throne.
Sansa has no real opposition to her brother becoming king. At first, she's in protective big-sister mode: she's concerned because she knows Bran doesn't actually want to be king, and his injury means he can't have children — and we've all seen what happens when kings don't have proper heirs.
Presumably he wants to locate and kill the dragon (or otherwise chain it up) so it can't burn down any more cities, though it's also possible that Bran wants Drogon near to be able to warg into him and control him (but that would sort of be more of the same, so probably not).
In Season 8, Bran reveals that the Night King wants to destroy him because he holds all of the history of man.
All of that changed when Bran entered the picture as the Three-Eyed Raven. This introduction gave the Night King a true motive outside the typical megalomania of Big Bads. By wiping out the Three-Eyed Raven, he would be wiping out all memories of the previous world and fully ushering in his new era.
Aside from being a symbol of death, the creation of the Night King was a way to add more history to the fictional universe. The series had the opportunity to dive deeper into the mythology of White Walkers by tying in their creation to the Children of the Forest and the First Men.
She said some people believe Night's King was a Bolton, a Magnar of Skagos, an Umber, a Flint, a Norrey, or a Woodfoot. However, she identifies Night's King as a Stark of Winterfell and brother to the King of Winter and suggests his name was Brandon.
No, the Night King is not a Targaryen.
In the books, however, the story of the Night King is slightly different. The Night King isn't the leader of the White Walkers. Instead, he's a legendary historical figure who supposedly betrayed the Night's Watch to marry a “corpse queen” and then crowned himself a king.
According to Old Nan, the Night's King may in fact be (hold on to your small clothes) a Stark! If that isn't enough, legend has it that the Night's King was not just any Stark, he was King Brandon Stark's brother—or possibly Brandon Stark himself!
Just as with the White Walkers under his control, the Night King was immune to fire due to the extreme cold he radiated. During the Wight Hunt, he was seen walking through a wall of dragon fire unharmed, his mere presence causing it to flicker and go out.
First many suggested that Daemon Targaryen was the Night King. Then there was a stir around King Viserys being a White Walker. Now that those theories were shut down, a new theory regarding Aemond Targaryen as the Night King has surfaced.
He cares for no one at all. If someone does not benefit him or is incapable of serving his ends, they are dispensable. This relentlessness - and unwillingness to prevent the tragedy that forges his path to royalty - is what makes Bran the true villain on Game of Thrones.
The Night King is a white walker, and it seems that there are only two things that they are not impervious to: Valyrian steel and Dragonglass (obsidian).
What do the White Walkers do with the babies that Craster gives them? They turn them into more White Walkers.
No, Hodor is not a Targaryen.
In the TV series, it's revealed that Hodor was originally a stable hand called Wylis who was essentially lobotomized when Bran attempted to warg into Wylis, connecting him with present-day Hodor who was holding back an army of wights.
Senseless, Bran dreams of his falling from the tower and of a three-eyed crow that offers to teach him to fly. With the crow's guidance, Bran wakes; but having been crippled by the fall, he is unable to walk.
Drogon is believed to be the reincarnation of Balerion the Black Dread, but Daenerys decides to give him a new name for his new life. The largest and most aggressive of Daenerys's three dragons, Daenerys has problems reining him in.
Dreamfyre laid a lot of eggs in her time, including three that were stolen from Rhaena by a woman named Elissa Farman. No confirmation, but there's a theory that these three stolen eggs are the ones that ended up in Daenerys Targaryen's hands, later becoming Drogon, Viserion, and Rhaegal!
Now, there are two theories here: One is that Drogon, a smart dragon, recognized what the Iron Throne symbolized and burned it, rebuking the desire for power that ultimately killed his mother. The other is that Drogon mistakenly thought that the pointy sword chair stabbed and killed Daenerys.