Worm in the Earth

From Overview:

While adventuring under the earth, the PCs chance across the stomping grounds an immense purple worm. The cyclopean annelid is in its element, able to attack from any direction and retreat into its tunnel network if injured – or fed with the body of an unfortunate PC! At levels where fly spells and level adjustment races are common, few foes can completely outmaneuver the party – something this worm can do better than most wyrms.