The creature is unconscious.
     The creature must make death saving throws.
     This condition ends immediately on the creature when it regains hit points.

Dropping to 0 hit points or fewer subjects an adventurer to this condition. Monsters normally die when they drop to 0 hit points, so this condition applies to them only in exceptional situations. See “Dying and Death".

Published in Player's Handbook, page(s) 277, Player's Handbook 3, page(s) 220, Rules Compendium, page(s) 231, Monster Vault: Threats to the Nentir Vale, page(s) 123.