Contains spoilers for the Lost Mine of Phandelver.
In the 5e Starter Set adventure, you encounter
a CR 3 Wraith (named Mormesk) (LMOP, p. 59). Its Life Drain attack reads slightly differently than the CR 5 Wraith in the Monster Manual.
I'm not referring to how their stats are different to reflect their CRs. Both of their attacks refer to the effect of reducing the target's HP maximum on a failed Constitution Saving Throw, but while the normal Monster Manual wraith's Life Drain says (p. 302), “The target dies if this effect reduces its hit point maximum to 0,” the specific enemy's Life Drain specifically says “If this attack reduces the target's hit point maximum to 0, the target dies” (emphases both my own).
So you can see that these are different. In one case a creature dies if the attack brings it to 0 HP (even succeeding on the Constitution saving throw), while in the other case the creature would still be alive at 0 HP (albeit unconscious) if the attack brought them to 0 but they succeeding on the saving throw.
Is this to make the monster in the adventure different from the monster in the MM, possibly to give the campaign a little flair, or is an error, possibly an inline update between the campaign and the MM? Does the Life Drain attack kill automatically instead of knocking out, or is it only on a failed save?
Best Answer
As far as I can read, in both cases the HP maximum is only drained if you fail your Con saving throw. If you save successfully your HP maximum stays the same, therefore it doesn't drop to 0 and thus you don't die from Life Drain (regardless of whether the raw damage still kills you).
The two instances of the text are worded a bit differently, but in both cases it states rather clearly that the dying automatically part only refers to the maximum HP. Note how, even in the adventure's wording, the attack only reduces the target's HP maximum if the effect kicks in due to the failed save.