A Banshee (MM, p. 23) has the Wail action:
Wail (1/Day). The banshee releases a mournful wail, provided that she isn’t in sunlight. This wail has no effect on constructs and undead. All other creatures within 30 feet of her that can hear her must make a DC 13 Constitution saving throw. On a failure, a creature drops to 0 hit points. On a success, a creature takes 10 (3d6) psychic damage.
If someone was already at 0 hit points (for example, having failed their saving throw against a different Banshee's Wail), what would happen if they fail their saving throw against this Banshee's Wail (assuming they still need to make a saving throw)?
- The unconscious creature takes one step towards death, as though they took damage (thus making the saving throw meaningless, since they'd take a step towards death either way);
- The unconscious creature just dies (unlikely, but in case there's something written that would imply this hidden in the rules somewhere);
- Nothing, since this isn't damage, its just an effect (meaning the unconscious creature would be better off failing this save than passing in this case!)
- Something else...