Tethys (also spelled Tethis) is a "sea troll" made entirely from ambulatory water. Trolls native to the Discworld are mineral products, but this alien troll seems to be an elemental (in the sense of "the 4 elements", not a periodic table element) instead. Through him, one can discern tidal patterns, and several different species of fish. In fact, due to the power of the Disc's moon, he suffers from chronic tides.
Tethys, a slave of the Krullians, is one of a large number of such slaves who perform the mammoth task of collecting anything useful which washes up against the Circumfence - the barrier that encircles more than 3,000 miles of the Edge of the Disc, and which nominally belongs to the Arch-Astronomer of Krull. He captured Rincewind and Twoflower when they were washed up against the Circumfence, and briefly held them captive until they were collected by a Krullian vessel powered by the hate of Hydrophobes.
Tethys is an alien to the Discworld, and the only sea troll on the Disc. He used to be a "meatman" (the water-world equivalent of a fisherman-- he hunted wild game on prairies) on his home world of Bathys, where he fell off the edge, froze solid, fell through space for some time and finally thawed out as he approached the Disc's Sun, and landed widdershins of Krull, where he was eventually captured and made a slave. He has considered jumping over the Edge to find his home world again, but has never found the courage.
Tethys' fate is unknown. It is presumed that he entered in the Krullian spaceship after Twoflower, but when Rincewind and Twoflower were brought back to the Disc by the Change spell, the sea troll wasn't there. Maybe he somehow found his way back to the world of Bathys and could live in peace ever after with his wives and children.
On Roundworld, Tethys has many meanings:
- The original Tethys was a Titaness in Ancient Greek mythology who gave her name to a moon of Saturn.
- There was also a Tethys Ocean that existed between the continents of Gondwana and Laurasia during the Mesozoic era before the opening of the Indian Ocean (relating to the Mediterranean sea and much of the current Sahara desert).
- Click here for the gastropod who glories in the name.
One can't help wondering whether TP would ever have written Tethys into existence by about book 5 - an elemental troll is not really in keeping with Discworld physics and chemistry, and he has never been mentioned since ...
Of course, Tethys is the only sea troll on the Disc and it's quite possible that "sea troll" doesn't mean "a troll from the sea" anymore than a prairie dog is a dog or a sea monkey is a monkey or a sea horse is a horse.
It is entirely possible that the Tethys/Tethis of The Colour of Magic is a later refinement of the intelligent water-based being Chatogaster, who is capable of assisted travel between planets, and represents a recycling of ideas between The Dark Side of the Sun (1976) and the Discworld series.