Tythonus was the native planet of the Tythonians. It was rich in metals and had at least some forms of plants, though they were becoming rare.
The Tythonians had hoped to trade with Chloris, but the first person their ambassador Erato encountered on arriving in his eggshell-like ship was Adrasta. She took his communicator device and trapped him in the Pit to preserve her monopoly in metal. Erato retrieved his communicator and killed Adrasta.
He warned the Fourth Doctor that the Tythonians had set a neutron star on a collision course with Chloris in retaliation for his imprisonment. With The Doctor’s help he was freed from the Pit. In space, he spun an aluminium shell around the star, allowing for it to be pulled off course by the TARDIS’ tractor beam and thus saving Chloris. (The Creature from the Pit