Shai-Hulud is the name given to the colossal sandworms that inhabit the enormous deserts of the planet Arrakis in the Dune novels created by Frank Herbert. These gargantuan annelids - growing as large as 450 meters in length - live extraordinarily long lives (no one knows exactly how long) and have a virtually indestructible hide. The Fremen - the tribal humanoid natives of Arrakis - rever the sandworms in a rather fanatical light, believing them to be gods amongst mortals and referring to them in such names and titles as "Old Father Eternity" and "Grandfather of the Desert".
The maw of Shai-Hulud is filled with thousands of crystalline teeth that can rend through rock and metal. They will devour pretty much anything in their path, though they only really gather nutrition from sand plankton and smaller sandworms. They are attracted by rhythymic vibrations and are a consistent threat to those who mine the deserts for the spice melange. Melange is a by-product of the sandworms' life cycle and has been known to extend human life and expand consciousness, even granting the ability to fold space. These properties have made melange the most valuable commodity in the Dune universe.