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. (HOMEVID: Shada)

Other Notable Prisoners
While searching for Salyavin, Skagra accessed the records of war criminal Rungar and mass-murderer Sabjatrik who were also prisoners in perpetuity. (WC: Shada, AUDIO: Shada) According to one account, Lady Scintilla was imprisoned in Shada for "conspiring with Carrionites". (PROSE: Shada)

Shada held other renegade Time Lords. 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. (PROSE: Prisoners of the Sun)

Grandfather Paradox was imprisoned on Shada. (Legend had it that he had been imprisoned rather than executed because "everyone was more afraid of him dead than alive".) Early during Romana's Presidency, she suffered an epileptic fit under the influence of the Carnival Queen and signed an executive order freeing three hundred prisoners from Shada, including the Grandfather, who led the freed prisoners. (PROSE: Christmas on a Rational Planet)

Prisoners in Shada had dragon tattoos which snapped upon their release. (PROSE: Christmas on a Rational Planet) Daniel Joyce had the tattoo of a prisoner of Shada. (PROSE: 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.