Condition Immunity to Unconscious

Question

There is a very similar question here, but it is asked and answered using 4e rules.

In 5e, Elementals all have the Condition Immunity to the Unconscious condition. We have been playing it that it confers immunity to spells and effects that would confer unconsciousness without necessarily affecting HP, such as the Sleep spell. I can't find a reference as to what happens when the Elemental drops to 0 HP through combat.

Most DMs follow the recommendation of the DMG and just have the creature die, since it is a monster. However, characters are given the option of knocking a creature out rather than killing it at 0 hp. And some DMs do indeed allow important monsters death saves.

What happens when a creature with this immunity drops to 0 HP? They cannot go unconscious. Are they incapacitated? Can they act?

Answer 1

Death, by virtue of no choice at all

If players are given a choice between outcomes, but one outcome is forbidden, then it defaults to the only available outcome.

Monster Manual, pp7 under Hit Points states:

A monster usually dies or is destroyed when it drops to 0 hit points.

Meanwhile, the Players Handbook, pp 197 states (emphasis mine):

When you drop to 0 hit points, you either die outright or fall unconscious

Now, they're clearly using "die outright" for when Instant Death occurs via remaining damage equaling or exceed hit points maximum. However, combining the two generally leads to the idea that the Monster Manual suggests death, while Players get unconscious or death. Since some creature are immune to unconscious, they would most likely just die.

A more narrative path

This, of course, doesn't mean you/your table can't find a middle ground. Maybe you don't want that elemental to simply die as they are a player in your storyline. That's fine! You can either 'reflavor' the Unconscious condition or come up with some reasonable substitute (maybe just Incapacitated?) There are also options of adding things like the Frenzy Barbarian DC check to stay at 1HP or other abilities that prevent creatures from going to 0 HP.