By request I'm DMing an adventure with a lot of H.P. Lovecraft overtones, and the party has encountered the Big-Bad-Guy tentacle horror, a twenty foot Obyrith I've styled after the Far Realms Glothoma. It's nearly out of hit points, and I'd like it to cast one final spell without a TPK, but I'm worried I've chosen the wrong one due to the flavor text.
The description of the psychic scream spell (Xanathar's Guide to Everything, p. 163) states:
Each target must make an Intelligence saving throw. On a failed save, a target takes 14d6 psychic damage and is stunned. On a successful save, a target takes half as much damage and isn’t stunned. If a target is killed by this damage, its head explodes, assuming it has one.
Could someone parse this out for me? If a character takes enough damage to hit zero hitpoints, are they "dead" as far as this spell is concerned, suffering the homage to the movie Scanners, with the full head popping effect -- or is dead only dead when they either fail enough death saving throws, or die instantly from the Massive Damage rule?
I ask because despite having two war clerics in their party, they're only level 6, which means the best they can respond with is revivify - which can't restore missing body parts:
You touch a creature that has died within the last minute. That creature returns to life with 1 hit point. This spell can't return to life a creature that has died of old age, nor can it restore any missing body parts.