Bayle the Dread is an optional Legend Boss introduced in Elden Ring: Shadow of the Erdtree. You don't need to defeat it to complete the game. From that zone, head to a path to the north.

The Bayle's Tyranny is an Incantation introduced in Elden Ring: Shadow of the Erdtree. One of the greatest Dragon Communion incantations. Harnesses the fearsome power of Bayle the Dread.