Shada

Shada was a planetoid (or asteroid) into which Time Lords built a prison for would-be conquerors of the universe.

History
The villainous Skagra managed a successful jail break of the prisoners while in search of Salyavin, the notorious Time Lord criminal who had developed a means of projecting his own thoughts into anyone's mind. Skagra believed that with this power, he could impose his own thoughts and will on the universe. Salyavin, though, had already escaped and was living a peaceful life as Professor Chronotis of the University of Cambridge. (DW: Shada)

Other Notable Prisoners
While searching for Salyavin, Skagra accessed the records of war criminal Rungar and mass-murderer Sabjatric - also prisoners in perpetuity. Apart from Salyavin, Shada held at least one other renegade Time Lord. The Master was imprisoned there, though he also escaped. This would happen just before the Third Doctor's first confrontation with him since the latter's exile to Earth. (VD: Prisoners of the Sun)

Daniel Joyce had the tattoo of a prisoner of Shada. (EDA: Unnatural History)

Behind the scenes

 * In the unaired serial, Shada was originally going to hold a Dalek, a Cyberman and a Zygon.
 * The webcast illustration of Shada's interior contains several Daleks, as well as Arthur Dent and Zaphod Beeblebrox from The Hitchhiker's Guide to the Galaxy as a nod to Douglas Adams.