The Princess With A Sacred Power is a character mentioned in the The Legend of Zelda: Breath of the Wild. This princess fought Calamity Ganon alongside the four Divine Beasts and the Hero Wielding the Sword that Seals the Darkness 10,000 years before the events of the game. During the course of the game, the story of the hero is related to Link by both the King of Hyrule and Impa.
Theory
Theory warning: This section contains theoretical information based on the research of one or several other users. It has not been officially verified by Nintendo and its factual accuracy is disputed.
It seems likely that the Hero Wielding the Sword that Seals the Darkness is an incarnation of Princess Zelda.
Theory warning: Theories end here.