A Tythonian communicator was a flat, heptagonal device with five panels and a centrepiece. If touched, the device used the larynx of whoever touched it to allow its corresponding Tythonian to speak. It could hypnotise people to have them return it to the user.
It could work from a distance and on robots, as K9 was used to communicate while Erato was outside the Doctor's TARDIS.
Erato's communicator was stolen by Lady Adrasta when he landed on Chloris. She then trapped him down a pit. Adrasta kept the communicator in her throne room, but it was stolen by bandits who raided it.
The communicator hypnotised Torvin and Edu, and had them take the communicator to Erato. With it, Erato was able to explain Adrasta's misdoings to the Fourth Doctor. (TV: The Creature from the Pit)