Tethis was a sea troll who was native to Bathys, another disc-shaped world, and was made entirely of water. He was a meatman, hunting wild animals across the prairies of his home. At some point, he fell off the edge of his world, froze solid, plummeted through space and landed on the Discworld. He was captured by the Krullians, and made a slave. He was assigned as a lengthman on the Circumfence, and captured Rincewind and Twoflower when they washed up on his length of the fence.