Shada

Shada was the prison of the Time Lord locked in a bubble outside the universe (PROSE: Shada) for would-be conquerors of the universe. (WC: Shada) According to one account of the Eleventh Doctor, it was a prison planet; (COMIC: The One) according to other accounts it was a planetoid (or asteroid) into which Time Lords built a prison. (PROSE: Shada, AUDIO: Shada)

Features
The prison was both locked in a bubble outside the universe and timelocked. (PROSE: Shada) It was contained by a quantum wall and its surface was patrolled by robot guardians. (COMIC: The One)

The prisoners of Shada were stored each in their own separate cryogenic cell, alive, but frozen in time, in perpetual imprisonment. (PROSE: Shada) They had dragon tattoos which snapped upon their release. (PROSE: Christmas on a Rational Planet)

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. (PROSE: Shada)

The Eleventh Doctor visited Shada in search of the Master's TARDIS. He entered by crashing the Then and the Now onto the quantum wall containing the prison. After landing, he remembered the planet's name. (COMIC: The One)

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 happened 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)

The Heretic was either killed by the Time Lords or imprisoned on Shada after plotting to regenerate the universe. (AUDIO: The Two Masters)

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, but the scenes set in Shada's interior were never recorded.
 * 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.