Dragon Age: Origins

Riordan reveals to the Warden on why Grey Wardens are the only ones capable of killing an archdemon to end a Blight: If it’s slain by anyone else, the archdemon will simply possess the nearest soulless darkspawn in the vicinity, rendering it effectively immortal. But if slain by a Grey Warden, who is tainted with darkspawn blood, it will be forced to possess them instead. And with the Grey Warden already possessing a soul, the process results in the mutual annihilation of the souls of the archdemon and the Grey Warden.

Example of:
Someone Has to Die