What happens when a creature with Condition Immunity to Unconscious drops to 0 HP?

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 Barbarian Relentless Rage save to stay at 1HP, or other abilities that prevent creatures from going to 0 HP.

Answer 2

Death, if it is "just a monster"

As the "Monsters and Death" section of the PHB explains:

Most DMs have a monster die the instant it drops to 0 hit points, rather than having it fall unconscious and make death saving throws.
Mighty villains and special nonplayer characters are common exceptions; the DM might have them fall unconscious and follow the same rules as player characters.

If the creature is narratively important, it will "follow the same rules as player characters", which are more complicated.

Otherwise, it can continue to act at 0hp

Player character rules are contained in the larger Dropping to 0 Hit Points unit. This begins:

When you drop to 0 hit points, you either die outright or fall unconscious, as explained in the following sections.

As the introduction to this unit, this is a general statement that these are the typical consequences of falling to 0hp - either death or unconsciousness. However, nothing here says that if you can't be affected by one, you must immediately have the other. Rather, this introduction is saying to apply the "following sections" to determine what happens. The "following sections" are (1) Instant Death, (2) Falling Unconscious, and (3) Death Saving Throws. They are meant to be applied in order.

Instant Death

Unless the blow that took you to 0hp did exactly enough damage to place you at 0, you have damage 'left over'. If this left over damage is equal to or greater than your maximum hp, you die immediately. If this does not apply, proceed to section (2).

Falling Unconscious

If you did not immediately die from the blow that brought you to 0, you are now unconscious. Proceed to section (3). If you are Immune to the Unconscious Condition, you would not be unconscious - but you would proceed to (3) all the same.

Death Saving Throws

This is the longest section, and explains how to make death saving throws.

Whenever you start your turn with 0 hit points, you must make a special saving throw, called a death saving throw, to determine whether you creep closer to death or hang onto life.

Notice that unconsciousness is not a precondition for section (3). You make death saving throws whenever you start your turn at 0 hp (and later we learn you also have a failed save applied whenever you take damage while you are at 0hp) - regardless of whether or not you are unconscious.

Thus, for a monster like an elemental with immunity to unconsciousness, and for which the DM wished to use the PC rules: (1) The DM would first check to see whether the elemental was killed from massive damage. If not, section (2) would be ignored due to the monster's immunity¹. Finally in (3) we would have a monster conscious and capable of action at 0hp. The monster would need to make a death save at the start of every turn, and would take a failed save every time it subsequently took damage, but otherwise it could keep acting at zero hp until it either died from failed saves, or was healed or spontaneously recovered. It would certainly not be incapacitated, as nowhere in the rules for operating at 0hp does it say that this causes incapacitation.

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¹And see also: Is an Immune creature considered to have the condition without suffering its effects?