Brynden Rivers

Ser Brynden Rivers was a greenseer and one of the thirteen bastard children of King Aegon IV Targaryen, his mother being Aegon's sixth mistress, Melissa Blackwood. He served as Hand of the King and Master of Whisperers to his nephews Aerys I Targaryen and Maekar I Targaryen, before joining the Night's Watch alongside his grand-nephew, Aemon Targaryen. Brynden eventually became the Lord Commander of the Night's Watch, but vanished beyond the Wall in 252 AC at the age of 77, and was quickly presumed dead. In actuality, Brynden met the Children of the Forest and became the penultimate Three-Eyed Raven. In 298 AC, he began communing with Bran Stark, eventually compelling him to travel beyond the Wall, with Bran arriving at his cave in 301 AC. Brynden trained Bran in his cave, before being killed by the Night King after he learned of his location. Bran succeeded him as the Three-Eyed Raven, and was ultimately successful in killing the Night King during the Battle of King's Landing. It's purpose served, the "Three-Eyed Raven" ceased to exist, and Bran regained his old personality.