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.

One of the greatest Dragon Communion incantations. Harnesses the fearsome power of Bayle the Dread. Channels the form of the dread dragon to emit a mighty roar accompanied by a heatwave burst ...