Do you use unkillable enemies?

[Senko]

Just something I'm wondering does anyone else use enemies like Baba Yaga who can't be killed or defeated until the party figures out and counters their power source ala sauron's one ring being destroyed to defeat him?

[Claxon]

I have, but usually it's forecasted and known to the party well before they would ever encounter them. Getting the McGuffin is part of the puzzle/plot of the story and if the party meets the BBEG before they do so it's usually pretty clear that at this point the enemy is plot and not meant to be fought.

[TxSam88]

this is a typical lich - which is encountered fairly often in RPG games - so yes.

[Mysterious Stranger]

Yes but it should not become common. This type of enemy should be the end of the story or at least the story arc.

[Diego Rossi]

Unkillable is different from unbeatable. I use unkillable enemies as a recurring nemesis, but they generally can be beaten, forced to retreat, thwarted, and so on. They are meant to be killed or banished for a century or more when the party level is high enough. That would be the cap point of the campaign.
Demon and devil lords generally, even if killed, reform after a time, same thing for lichs, and plenty of other opponents. Even several PC have capstones that allow that.

[Bjørn Røyrvik]

Unkillable =/= difficult to kill/defeat

Truly unkillable would be an absolute situation, the GM just saying 'nope' to absolutely everything the players try. I haven't used any of these yet.
If you only mean 'hard to kill, possibly requiring some special circumstance or material', then yes - I have used loads of them. The rule books, adventures and settings are full of them. This is a relative situation, what is hard to kill for one person may be trivial to another . Having lots of HP and good AC might make you basicaly unkillable to certain creatures. Having a bunch of magical protections can make you basically unkillable to some people.

[Mark Hoover 330]

So like, vampires and liches? Those are in my games all the time. I also made up The Swarm Lord: a sentient being made up of various Swarm types who reforms after 3 rounds at full HP again unless you destroy the hives and hatcheries embedded in the walls of his chamber.

I ran a megadungeon once like 10 years ago based around the video game Gauntlet, so PCs would occasionally find "spawners;" devices that would pump out more of a type of monster at random intervals. The players hated it and the campaign ended after only a handful of sessions.

For the most part though most foes, even super important BBEGs in my games have a fixed number of HP I don't deviate from and they are destroyed/killed/defeated when that number either hits zero or lower. I find that players rise to the occasion of rare instances where foes are "unkillable" but tend to get frustrated quickly with multiple applications of it.

Oh yeah, and players also don't like it when they take for granted that monsters are slain when they drop unconscious but then you as GM have several of those monsters ignored by the PCs stabilize, heal back to positive HP, level up after cheating death, and then become elite foes with a specific grudge against the PCs, all because the players didn't say the words coup de grace.

[Azothath]

in a play or drama some characters represent ideas/themes/forces that are beyond human control or grasp. Those are essentially "unkillable".
There are deities.
You also see this trope or style with Mythos creatures where simply creating a holding pattern is considered a win.
Undead, constructs, a few outsiders or aberrations essentially "fall down" or are destroyed as they aren't really "alive" per se.

In a game I think it is best to keep these generalized ideas or representations somewhat at arm's length from the PCs. Players in a simple way want to show their prowess by dominating or subjugating NPCs/monsters and this isn't really possible with these characterizations. They can martially defeat the representatives or lackeys of the 'powers'. It also moves the game towards a more dramatic or mythic level which naturally happens as levels approach 20th level.