Grandfather Paradox

Grandfather Paradox was the nominal leader of Faction Paradox.

Biography
When the Grandfather was in his youth, he travelled back in time into his own past and used an ordinary knife to murder his own grandfather. The paradox of this caused the Grandfather to be simultaneously alive and dead — murderer and victim. A consequence of the paradox was that the Grandfather only had one arm. (PROSE: The Gallifrey Chronicles)

Called the "voodoo priest of the House of Lungbarrow", Grandfather Paradox was imprisoned in Shada. Popular fable maintained that he was imprisoned rather than executed because "everyone was more afraid of him dead than alive".

When the Carnival Queen threatened rationality throughout the universe, Lady President Romana had an epileptic fit during which she signed a release order for three hundred prisoners, including the Grandfather. (PROSE: Christmas on a Rational Planet)

According to Faction Paradox legend, the Grandfather Paradox had one arm. He had removed this arm to remove the Time Lord branding/tattoo of a prisoner, and he would have committed the ultimate paradox.

The speaker's chair in the Eleven-Day Empire was left vacant to be taken by the Grandfather upon his appearance to the Faction Paradox. (PROSE: Interference - Book One) The Faction Paradox held the Grandfather Paradox up as one of the ultimate forms of temporal paradox.

Resulting from a corrupted timeline, where his biodata was corrupted by Faction Paradox, a future version of the Eighth Doctor modelled himself as the Grandfather Paradox; in this iteration, the Grandfather was a bald man with a stump in place of his left arm wearing black armour and robes, described by Fitz Kreiner as resembling the Doctor 'if he'd spent twenty years in the navy before becoming a psycho'. (PROSE: The Ancestor Cell) Other accounts suggest that Grandfather Paradox appears as a twisted, morally bankrupt and broken version of whoever is looking at him or "the ghost of Christmas cancelled" as the Doctor snidely put it. (PROSE: The Gallifrey Chronicles) However, if the Doctor was indeed the true Grandfather Paradox, then this version had indeed committed the ultimate paradox; he cut off his arm not to remove the Time Lord branding/tattoo, but because he knew it was the arm which the Doctor would use to destroy Gallifrey. The timeline where the Doctor became the Grandfather was erased when the Doctor's TARDIS contained the infected timeline in itself until the Doctor drained its power, forcing the universe to 'choose' one timeline or the other, with history 'choosing' the world where the Doctor was never infected, erasing the Faction and the Grandfather from existence. (PROSE: The Ancestor Cell)

Behind the scenes

 * The name Grandfather Paradox punningly refers to from theoretical science and science fiction.
 * Grandfather Paradox was intended to never actually appear, similar to the Doctor's true name, or Judge Dredd's face.