There are three dog-head markings on the wall across the map: in the Infirmary, in the Docks and on the Broadway, under Electric Cherry.
A head resembling a Hellhound will spawn on the wall when a zombie is killed near one. Cerberus will eat zombies killed near it (although if it is already eating one, it will not take a zombie killed during the eating process), and after it has eaten six zombies, it will vanish into the wall. After feeding each of the heads six zombies, a wall in the Citadel Tunnels will open, revealing the Hell's Retriever. One of the three heads is also featured in Cell Block, in the starting room (where the B23R is bought). Since the Hell's Retriever is not available in Cell Block, feeding Cerberus' head enough zombies will make a random power-up spawn and rise out of the lava pit below.
Three Cerberus symbols in front of Hell's Retriever spawning room.
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The player will take very slight damage when touching one of the heads.
Cerberus pauses for a moment after devouring a zombie.
Cerberus will not eat zombies that are killed while it is already eating one.
Zombies killed with the Zombie Shield or Nuke cannot be fed to Cerberus. In addition, it will not eat crawlers (zombies with dismembered legs) or Brutus.
There are three heads, each one must be fed six zombies, which is a possible reference to the number of the beast "666".
Above the entrance to the Hell's Retriever room is a piece of sheet metal with three dog-head markings. These markings can also be seen on the walls where a head will spawn from. For every head fed, one of the three head markings will light up. Once all three are lit, the wall will disappear, allowing access to Hell's Retriever.
Cerberus is listed as "dream_catcher" in the game's files.