Bayle the Dread is easily one of, if not the, hardest boss in Elden Ring: Shadow of the Erdtree. He's optional—which is a red flag if you know FromSoftware games—and the first time you hear about him is from a man with no legs screaming his name over and over out of a relentless thirst for revenge.
As far as dragon bosses go, Bayle is brutal: He may lack arms and a leg after years (centuries?) of conflict, but he's still an agile dragon who can spit lightning storms and set his arena ablaze as you face off against him.