The Dark Knight

Joker, made himself immortal, just in time!

The Joker: Oh, you. You just couldn't let me go, could you? This is what happens when an unstoppable force meets an immovable object. You are truly incorruptible, aren't you? Huh? You won't kill me out of some misplaced sense of self-righteousness. And I won't kill you because you're just too much fun. I think you and I are destined to do this forever.

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Batman: What did you do?
The Joker: I took Gotham's white knight and I brought him down to our level. It wasn't hard. You see, madness, as you know, is like gravity. All it takes is a little push!