Gnarl
From Ardrana
Gnarls are a type of magical creature somewhat similar to gargoyles. They are generally left as guardians of a place or item, and until they are activated appear to be simple cubes made of metal. When a triggering event occurs, however, they quickly transform into terrible humanoid creatures, immune to any metallic weapon.
Though these creatures are very rare, the secret to their creation was known, at least, to the wizard Bargle.
Note: Gnarls were the creation of Michael Ritskowitz.