The Tythonians were natives of Tythonus, notable for their huge size and longevity of life. They could weave metals from their bodies.
The Tythonians were massive, blob-like organisms, sometimes hundreds of feet long. They were glowing green and had an outer membrane that was deeply creased. They had no true limbs, but had two large pseudopods. One pseudopod was shaped like the letter Y, while the other was simply a large tube. They had no vocal cords and communicated with the aid of Tythonian communicators. Tythonians could live for forty thousand years, maybe longer if they didn't move continuously.
Tythonians subsisted on chlorophyll and mineral salts, which they could absorb through their skin. Their mineral rich diet allowed them to weave a special, extremely tough material from their tube-like pseudopods. This material was self-regenerating and could fulfil a variety of functions. (TV: The Creature from the Pit)
Tythonian technology was usually based on materials that they could naturally weave. Erato travelled in a massive egg-like craft of this material, with a photon drive made of other metals. Tythonians could also control the paths of neutron stars, which they used in times of war. Once they manipulated the path of a neutron star, they would then direct it towards a star in a solar system where their enemies were located and destroy them.
At some point in history, the planet Tythonus was running out of chlorophyll for food. The Tythonians discovered that Chloris was rich in plant life and could use the metals of Tythonus. The Tythonians sent Erato to try to set up a trade agreement, but he was captured by Lady Adrasta and was trapped in a mine for fifteen years.
Erato was eventually saved by the Fourth Doctor, but the distress signal of his spaceship was picked up by the Tythonians, who launched a neutron star at Chloris' sun in retribution. With the help of Erato, the Doctor was able to save the planet and set up a trade agreement. (TV: The Creature from the Pit)