Feed the Beast is an achievement/trophy in Mob of the Dead. It requires the players to feed the three heads of Cerberus throughout the map. To feed them, simply kill a zombie within the vicinity of a head; once it is full, the head will disappear, leaving its outline glowing. Each head needs to eat a total of 6 zombies to become fed and disappear, granting the achievement/trophy to every player currently in the game. Once all three have been fed, the Hell's Retriever will appear in the Citadel Tunnels behind a wall that was originally blocked. It can then be picked up and used. All players are allowed to use the weapon once it has been unlocked.