Now that's an idea for an impossible to slay main foe in a story: Slaying him just results in the person who struck the mortal blow transforming into a new version of the villain. To make it really nasty, each new iteration adds the power of the previous slayer.

This, of course, gives a solid excuse for the classic bit of having to bind or banish the threat. Leading to the new story, set generations later when he breaks free/returns and people don't realize why all the texts say that he cannot be killed. After all, everyone in town saw him fall from the sky as the band of heroes defeated him at his mountain lair....

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