Doom

You whisper a prophecy of imminent death, and these words slip into the threads of a creature's fate and cause it to pull toward the moment of its death. One creature that you can see within range makes a Charisma saving throw. On a failed save, you can take a @variantrule[Reaction|XPHB] when the creature takes damage from an attack. Don't roll the attack's damage dice for the damage. Instead use the highest number possible for each damage die. At the end of each of its turns, the target repeats the save, ending the spell on itself on a success.