What multiple-phase boss fight has your favourite story that develops within the fight itself, and not in the lore?

Say for example we have Ludvig. He's a big, scarred, deformed creature. A broken shallow of his old, proud self. We feel sorry for him, and are almost happy when he "reawakens" his old, more cognitive self. He stands tall, and proud again.
But it's a double-edged sword. He is now even more dangerous than he was before.
And when finally you bring him down, he gets to die at his best again. He goes out proud.