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.