The Guardian is the protector of the Aztec Crystal Skulls. It is an invisible, poltergeist-like force that will hunt down any thieves or basically anyone that threatens to use their power for purposes beyond their designation.
The original legends tell that this nightmarish entity is a servant of Tezcatlipoca, assigned by his master to guard the skulls of Quetzalcoatl. Carvings in several pyramids depict a massive black humanoid with menacing horns and glaring eyes. Despite these depictions, the Guardian is completely invisible, or perhaps it has the power to become invisible.
Signs of its presence include ravaged environment, loud, unearthly roars, and darkness. It also causes crimson tendrils to overrun areas where it passed. It is ruthless in its quest to regain stolen skulls and will kill anyone the skulls, or their owner, had contact with.
While it is thought that this entity is an instinctive beast, it shows signs of intelligence. When chasing the skull holder, it gives off a feeling of terror that frightens its prey, probably a tactic when going in for the kill and reveling every moment of it. It also interacts with other beings and manipulates them to its whim. For example, it might let loose deadly piranhas if the target attempts to swim downriver.
The full range of its powers is unknown, but it is speculated to have near-infinite power in order to find and reclaim the skulls.