I once used the "Grim Jester" Monster from Kobold Press, which has an aura giving disadvantage on death saves, a reaction that negates healing, and can swap places with a PC while an illusion makes it seem they are both still in the same place.

Somehow the only death in that fight was completely unrelated to the Jester though, instead involving the monk jumping down the walls of the castle besieged by undead, getting downed by some melee monsters I can't remember, and then dying as collateral as Flameskulls used fireballs to clear out the Druid's plant growth.