Thas are beings that dwell in the underground realms in various places in the Witch World. They are creatures that serve the Dark, though they can be neutral when things go against them.
Appearance: Thas have small, wry bodies with bloated stomachs. Their bodies are completely covered by root-like hair. They have muzzle-like faces not unlike a hound's that show fangs of teeth. Their eyes are pits in rounded skulls. They have claws that may be poisonous.
Life Span: The lifespan of a Thas is unknown.
Power: Thas have phenomenal power over the element of earth. In Gryphon in Glory they are literally able to churn the earth and suck in Joisan. Why they can't do this with the White Road is not explained, though the Road may be enchanted against their influence. They do not appear to be able to affect stone with their powers directly. They may be able to use their power over soil and their root-like plants to crack stone, undermine it and move it to create their tunnels but this probably takes time.
Other: Thas seem to be cowards unless driven by a more powerful evil force. They may have been created from human-stock by some Adept who was curious about what lay beneath the surface of the earth. It is unclear if they've always served the Dark or if they were created neutral and simply allied themselves with it, but given the possible age of the water trap found at the ruins in Warlock of the Witch World they've at least been direct enemies once before. They seem to have a strong aversion to water, though whether this is because of their nature or because they serve the Dark is unclear.