Whilst stated to have originally been a tribe of humans, since their banishment underground millennia ago the Worms have devolved into a semi-reptilian race of creatures who fear the light. Their skin appears to be scaly, they have glittering yellow eyes, and they move with the sinuous grace of a serpent. The Worms are capable of burrowing beneath the earth at speed, and their excavations are said to stretch for uncounted miles beneath the surface of mainland Britain.
After a great war with the ancestors of the Pictish tribes, the beings who would become the Worms of the Earth were exiled below ground, where they slowly devolved into the creatures they are today. Over the years, the surface people forgot about them, all but a few of the purest descendants of the old tribes, although the places which gave access to their underground tunnels were shunned by men.
During the Roman occupation of Britain, a Pictish king named Bran Mak Morn swore vengeance against a Roman governor who had brutally executed one of his people. Knowing that he could not achieve this goal alone, he bargained with a half-Worm hag for the location of an entry to the Worms' realm. Stealing the Black Stone, an artefact which held great importance to the Worms, Bran then agreed to return it to them if they would bring him the governor alive. This the Worms did, but when presented to Bran, the man was gibbering and insane, having been driven mad by what he had seen during his trip through the tunnels of the Worms.