I have a faint memory of a Dungeons & Dragons third edition creature with a trait that if you kill that creature, you become that creature.
The idea is that the creature possesses some kind of dying curse or death throes trait which makes it difficult to defeat. Whoever lays the killing blow on the creature is unwillingly and automatically transformed into it. It's not a benefit, reward, title, template, or form of lycanthropy. It's not Savage Tide Demogorgon where killing him makes you Prince of Demons. It's not the reverse where the body becomes the killer (e.g. sivak draconian). I don't know if there's a saving throw or a way to protect against it.
I vaguely recall an anecdote about a group who killed this creature, only to find that the party member who dealt the killing blow transformed into it, so they were back to square one. The party's proposed solution was that the paladin would allow himself to be bound in shackles and somehow make the killing blow, perhaps by some kind of passive damage effect. I don't remember whether the plan was successful.
I probably read or heard about it around 2000-2006. It was definitely D&D 3.0 or 3.5, which were the only RPGs I played at that time. I feel like it was probably a very high-CR fiend of some sort, although I may be confusing it with another creature which shared damage taken with enemies nearby. If it was in a book I read, it may have been one of the Monster Manuals or Dragon magazines. However, the notion of a boss enemy whose killer inevitably becomes it also appeared in a certain popular video game RPG released in 2001, and given that I recall an anecdote about another group rather than facing this creature personally, it's not impossible that this was some DM's game-inspired homebrew.
Did such a creature appear in any D&D 3.0/3.5 source?