Fatal Wounding (4e Variant Rule)
From D&D Wiki
In some cases, an attack has the good fortune to deal a heavy blow to a target. When this happens, it can be defined as Fatal Wounding. Under this definition, a target that takes a large amount of damage from an attack has a chance of being slain. For this purpose, a creature who takes its bloodied value in Damage (Even if the target is at maximum Hit Points) must as an immediate action make a Saving Throw. On a success, the creature is slowed from surviving such a massive attack. If the creature fails, it immediately drops to 0 Hit Points and is knocked prone. For all intents and purposes, if the creature is a monster, that creature immediately dies. Characters who fail a Fatal Wounding are unconscious, at which point standard death and dying rules apply.