Orcus's "Voice" lair action is casting the spell Power Word Kill. If he were to use it on a PC, would they drop to death saving throws or instantly die?
Spells do exactly what they say, whether cast on PCs or other monsters.
Power Word Kill says a creature with 100 hit points or fewer dies. PCs are not exempt from this as they are also creatures and thus are perfectly valid targets for the spell. Do not pass go, do not collect $200, do not make a saving throw vs the effect, do not drop to 0 HP and start making death saving throws. They just die like any other creature targeted by the spell.
Note that this is a 9th level spell so it's meant to be powerful.
Power Word Kill does kill a PC with fewer than 100 hit points without a save. However, a quick calculation shows that there would be many high level PCs that have more than 100 hit points at full health. For example, a level 18 paladin with CON 10 would have an average of 103 hit points.
Additionally, this spell is actually a bit on the weak side for a 9th level spell. If a monster cast Meteor Swarm on a party of PCs, they could take 40d6 damage each--a situation that could be just as (or even more) problematic as the instant death of a single party member.
"Kill" means they simply die. No saving throws, no death saves; the target is rendered dead.
Keep in mind that it counts current hit points not maximum. So damaging a creature and then using this spell is a viable tactic.
It also instantly kills any shapeshifters who have shifted and have less than 100 hp. Since such shapeshifters usually return to their normal form when dropping to 0, this does not trigger that because it deals no damage.
It also bypasses all types of resistance, both to magic and to damage types. Same for legendary resistance or feats (e.g. Lucky) or class features (e.g. the Divination wizard's Portent).
This is what makes it a great spell.
It is negated with the spell death ward, which negates it and then ends. Or alternatively, since power word kill targets "one creature you can see within range", you can avoid being targeted by either blinding the caster or by becoming invisible.