What is this creature whose slayer becomes it?

Question

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?

Answer 1

The Fafnheir, Father of All Linnorms (From a 2011 Pathfinder 1e regional book).

On killing, one triggers his Death Curse ability:

Death Curse (Su) Fafnheir is a difficult creature to slay, especially since he lives on in the body of any creature that slays him. When a creature slays Fafnheir, it becomes afflicted by the Curse of Fafnheir.

Curse of The Fafnheir: save Will DC 32; effect creature’s sense of self erodes as its personality is slowly replaced by Fafnheir’s—this manifests as 1d6 points of Charisma drain every 24 hours. A target whose Charisma drops to 0 becomes comatose and must immediately make a DC 32 Fortitude save or die; every 24 hours that passes thereafter, the victim must make a new Fortitude save to avoid death (unless its Charisma score rises above 0, at which point it takes 1d6 points of Charisma drain). If a creature dies from the effects of this curse, its body explodes in a 60-foot burst of burning wind, with effects identical to Fafnheir’s breath weapon. This effect occurs if the cursed victim dies from any effect, not just from the curse. One round later, Fafnheir gains the effect of a true resurrection spell, appearing at the same spot where the cursed victim died (or the closest area large enough to contain the Colossal creature), with full memories of the cursed victim’s doings and accomplishments while cursed. The only way to permanently slay Fafnheir is to avoid becoming cursed after killing him, or to remove the curse before the victim dies. The effects of this curse end prematurely and immediately if Fafnheir is restored to life by other means. The save DC is Charisma-based.

You might be confusing Pathfinder with d&d

Answer 2

Might be something like this, from the Age of Worms Adventure Path.

Ancient Night Twist

CR 20

NE Huge plant

Monster Manual III 110

...

Death Curse (Su)

Killing an ancient night twist invites a curse similar to that bestowed by the nightmare spell. The creature dealing the death blow must make a DC 28 Will save or suffer horrific nightmares each night, suffering 1d10 damage and becoming fatigued and unable to prepare spells for the next 24 hours. The nightmares continue until the curse is removed. Even after the removal of the curse, the victim remains fatigued for 24 hours afterwards. A limited wish spell or more powerful magic cast while the subject is in the throes of a nightmare is the only way to remove a night twist's death curse. If a victim of the death curse dies before the curse is lifted, a new ancient night twist sapling appears at the gravesite.

I believe there should be some (if not a lot) monsters with Death Curse ability back then, but it is unlikely for the curse to occur immediately rather than being a long-duration effect to give PCs enough room to notice and handle it (otherwise, it would be a guaranteed death which seems to be a bad game design).

This campaign (Age of Worms) was published in March 2006 under D&D 3.5, by the way.

The original, weaker, non-ancient version can be found here, which is CR 12 and has the same ability in lower DC: https://www.realmshelps.net/monsters/block/Night_Twist

PS. For this particular example, I wonder what would happen if the victim is raised or by any other means revived (or even become a zombie or skeleton) in a short time after their death, which surely leaves no "gravesite" behind for the night twist to grow.