Little is known about the Tholions. They occupy a planetoid called Tholos with an orbit very near the Sun. They have achieved space faring technology and their ships have been seen as far out as Neptune. Their ships do not employ standard Ætherdrive or steampulse Mechanology but instead rely on some sort of field generation effect.
A crew of dead Tholians were discovered in the asteroid belt after their ship had been involved in an unfortunate accident. Scientific investigation parties from the HMS Intrepid discovered that, Tholions live at a temperature of better than 500 degrees. Their bodies are composed of crystalline material and energized gasses.
After making first contact the Tholians made it clear that they wished to have no further contact with any other life form infesting the solar system. A mutual non-interference treaty was signed with the Tholions in September of 1982.