Is the Spirit Troll unable to be knocked out unless it doesn't regenerate?

Question

So I've seen some rules referencing that whenever a creature drops to 0 HP it is either knocked unconscious (and starts making death saving throws, unless it's already stable) or it dies outright.

However, trolls have the Regeneration feature that will let them regain hit points and which suggests that they never "die outright" unless the conditions are met that prevent its regeneration (typically fire or acid damage). Normally, from what I understand, a troll gets knocked out when it drops to 0 HP, but unless its Regeneration is stalled, it just regains hit points at the start of its next turn and can get right back up and keep fighting.

The Spirit Troll (Mordenkainen's Tome of Foes, p. 244) has a Regeneration ability that is nullified if the troll takes psychic or force damage since the start of its last turn. However, it also has immunity to the Unconscious condition.

Does the troll ever get knocked out from dropping to 0 hit points? Or can it act normally?

In my specific example, I ran a game where a Spirit Troll attempted to attack the group's fighter (Battle Master archetype), who Riposted the first attack when the troll missed and dropped the creature to 0 HP. As far as I could tell by RAW, the troll was still allowed to make the rest of its attacks on its turn.

I know it's ultimately up to DM discretion, but the whole thing sparked a discussion with my players (because I was honest with them and admitted to them that the troll was at 0 HP and told them I didn't know exactly how it should be ruled), and we ended up letting the troll make its attacks because the players really weren't struggling with the encounter that much anyway.

Answer 1

The Spirit Troll cannot be knocked Unconscious by any means

As you have said, the Spirit Troll is immune to the Unconscious condition. The rule for falling to 0 hp says:

If damage reduces you to 0 Hit Points and fails to kill you, you fall Unconscious (see Conditions ).

But the Spirit Troll is immune to the Unconscious condition, so simply reducing it to 0 hp has no effect on it.

Answer 2

On the start of the troll's turn, it has n HP.

It attacks and misses.

As a reaction, the fighter ripostes and does n damage.

It's still the troll's turn, it's at 0 HP.

The DMG says, "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"; however, if the DM wants to stay true to the spirit troll's stats, then the spirit troll can't die.

What to do?

One solution is that the spirit troll finishes its turn fighting at 0 hp. I don't like this one because the spirit troll doesn't have a feature that says it fights at 0 HP.

Another solution is that the spirit troll "dies", during its turn, as would be expected with another monster. The DM can choose to have it do a death saving throw or not as the DM wishes, per the monster death rules. Of course, it's not alive anyway. It has "a nonphysical form" with "a body as insubstantial as shadow". So when it hits 0 HP, it's insubstantial form insubstantially falls to the floor with insubstantial guts falling out or whatever. THEN, at the beginning of its next turn, assuming it did not take psychic or force damage during the turn it "died", it regenerates, regains hit points, and comes back to "life". Per its stats, it only actually "dies" - disappears, discorporates, disperses - "if it starts its turn with 0 hit points and doesn’t regenerate".

Answer 3

By RAW, specific wording of the Regeneration ability beats general.

I would rule it as the Spirit Troll is still standing (and presumably willing) to fight at 0 HP. Do note that the Spirit Troll cannot be knocked prone as well (immunity to Prone condition), therefore it does not fall prone due to the Unconscious condition (again, due to immunity to Unconscious condition).

So unless the party is able to stop the regeneration ability of the troll, the troll is able to fight on at 0 HP (and at the start of it's next turn, regenerate 10 HP!), which can be very dangerous if your players do not have the means to stop the troll from regenerating.