Tuba Samurai Boss

(just going to write some BS and you people can refine it)

The tuba samurai boss is among the top bosses in the game, rivalling the final bosses in speed and power. The first phase though, the samurai gives relatively slow and forgiving attacks, so that both you and him can get used to each other's attacks, for an honorable battle. As you progress, his attacks seem almost instant, with damage enough to one shot you. If you manage to kill him, he kneels over, only to unleash one last wide and fast attack, that kills you if you don't manage to dodge, parry, or block. After, he collapses, and you can choose whether to kill him or not. He can show up later to either help you or harm you if you spare him.

You can find him under the giant trumpet tree, behind the king's graveyard, awaiting a worthy maestro to finally defeat him.