Does Vecna’s Rotten Fate ability kill a player outright if it drops them to zero hit points? The ability states that the player will rise as a zombie on Vecna’s next turn, which implies that three rounds of death saving throws can’t happen. OTOH, I would expect the ability to say that character death happens immediately if that were the intent. This is from the Vecna stat block found in the Vecna Dossier and also in the Vecna: Eye of Ruin adventure. The ability in question is:
Rotten Fate. Vecna causes necrotic magic to engulf one creature he can see within 120 feet of himself. The target must make a DC 22 Constitution saving throw, taking 96 (8d8 + 60) necrotic damage on a failed save or half as much damage on a successful one. A Humanoid killed by this magic rises as a zombie at the start of Vecna's next turn and acts immediately after Vecna in the initiative order. The zombie is under Vecna's control.
Best Answer
It is Vecna’s next turn after the humanoid dies, not Vecna’s next turn after using the ability.
The part of the ability that raises the dead humanoid as a zombie doesn’t occur until the humanoid dies. So “Vecna’s next turn” in the ability description is not “Vecna’s next turn [after Vecna uses this ability]”, but rather it is “Vecna’s next turn [after the humanoid dies]”. As there is nothing else in the description to indicate instant death or forgoing death saving throws, the creature becomes unconscious as usual after reaching 0 hit points and must make death saving throws, unless the humanoid is instantly killed per the massive damage rules:
And finally, monster traits that do bypass death saves and kill player-characters immediately explain that in the trait description, such as the Reduced-Threat Wight’s Thrown Urn: