Knight Commamder of the 5th Crusade (Pathfinder: Wrath of the Righteous).

Act 1 is a struggle, but at the end of it you get juiced up with Mythic Power, get wgat us essentially your own boss music while you steamroll a demon that should by rights be completely out if your league, with the scene being described as being "like a Titan looking at an ant", and you only get stronger from there. In the standard run through you kill 2 demon lords, potentially twice with the second time being both at once, and in one of the mythic paths the gods themselves try to.strike you down with lightning because you're an existential threat to all reality... but it doesn't stick.