Now Bran has grown into a three-eyed crow, and can flash back to any place in the past time and space just by thinking, witnessing what happened at that time. In the seventh season, he went back to the past, witnessed Littlefinger's crimes, and witnessed the wedding scene of Regal Targaryen and lyanna stark, which really hammered Si Nuo's true identity.
Moreover, in the process of flashback, Bran can make a sound to hint and even control others with consciousness. This is reflected in several important flashbacks in the sixth season: after the battle of Blissful Tower, Bran called "Dad" to Ned Jr., and Ned turned around when he heard it, but Ned didn't care because he couldn't see Bran; In the process of flashback, Bran controls the present time and space, helps him escape (personally, it is the most touching scene in the sixth season), and turns Ado from a mentally sound teenager into a stupid life.
Even more surprising, Bran found the alien legion in his private flashback, but the Night King found him and grabbed his arm. Bran woke up only to find that the arm caught by the night king left a magic mark. It was through this mark that the night king found Bran and lifted the protective magic of the Three-Eyed Crow Cave. Although this can only be guessed, Wang Ye and Bran have similar or identical abilities, and we can't draw a final conclusion, flashback can definitely have an impact on the past and the present. Is there a plot that looks like the butterfly effect in the movie?
The point is, the three-eyed crow once warned Bran: "Although the scenery under the sea is beautiful, you will drown if you stay too long." This sentence means that if you stay in flashback time and space for too long, or want to change what happened in the past, you may be trapped in flashback and never come back. At the end of the seventh season, the night king broke a gap in the Great Wall with a frost dragon, and the ghost king army went south. If the human army goes to war with the alien legion, it is tantamount to throwing eggs at stones. Let's assume that in the eighth season, when human beings were desperate, Branshan went back to the past and possessed a young human warrior to persuade the son of the forest not to create an alien ghost, but the son of the forest caught him and controlled him, turning him into an alien ghost, that is, the birth scene of an alien ghost he saw in his flashbacks.
After being made into an alien ghost, Bran was trapped in the alien ghost as a human consciousness, and as a three-eyed crow, his consciousness crossed back. This also explains why the night king can see Bran when he flashes back, because they are alone.
As an alien ghost, Bran thought that as long as he didn't flash back to stop the birth of an alien ghost, there would be no alien ghost, so hundreds of people in front wanted to kill Bran. In order not to be killed, Bran, as a three-eyed crow, can only travel through different time and space and use various methods to stop ghosts. This also explains why there are so many brandon stark in history. These brandon stark made such things as building the Great Wall and giving land to the night watchman.
During the reign of Iris II Targaryen, the Targaryen family's monster killer dragon became extinct. Bran, the three-eyed crow, wants to control Iriz to breed dragons, but because the Targaryen family has magical properties, it can't be realized. However, the voice in Iriz's ear drove him crazy and was called the mad king. During this period, another brandon stark was born, because regal eloped with Leona, which angered the mad king and triggered the usurper's war. The later story is what we know.
If this hypothesis is successful, it explains why the night king didn't kill benjen stark directly and why he didn't kill Snow, because the night king was also a Stark, and Stark didn't kill Stark; Why does he know that dragons can destroy the Great Wall and its magic? Because he is brandon stark who built the Great Wall, and brandon stark has always been a person in history.
Of course, this is just a bold idea based on the existing plot. What is the truth? We still have to wait for the answer of the eighth season of this series.