H'Calos looked like a monstrous centipede with only six legs at the very beginning, the first pair resembling a praying mantis's. It was covered in a black chitin shell, and it had a four-hinged jaw and only two glowing green eyes.
The Star Worm was trapped in a magical slumber by the people in the Ixtzul valley, and was trapped beneath the great plaza in the city of Ixtzul. When awake, however, H'Calos could burrow through earth and stone, and could sense vibrations in the ground. It was more intelligent than an ordinary animal, but interested in little besides eating.
H'Calos attacked by bursting out of the ground and either attempting to swallow a creature whole or rip it apart with its forelegs. It had a slight magical resistance, and was immune to poisonous gas and fire.
H'Calos first fell to Maztica encased in a meteorite, landing in the valley of Ixtzul. The chieftain Osctl Sleeping Turtle found the creature first intending to make a bargain with it, but it somehow ended up bound in a enchanted sleep instead. However, Osctl's tribe still claimed that he had made a bargain with the creature: they would protect its sleep, and it would protect them, making it the guardian of their city. The threat of such a mighty creature helped the Ixtzulians gain power within the valley, and protected them from attack.
The great plaza of the city of Ixtzul was built over its sleeping place, and a great temple-pyramid was built to it as well. However, many stelae were also built to keep up the wards that kept it asleep. The Revered Counselors eventually came to consider themselves as wardens of the creature's prison as well.