Grandfather Paradox

Grandfather Paradox - also known as "the voodoo priest of the House of Lungbarrow" (PROSE: Christmas on a Rational Planet) - was the mythical founder and nominal leader of Faction Paradox. Even if a historical Grandfather "existed," he voluntarily stopped ever having existed, and was never under any obligation to be true to the legends about him. (PROSE: The Book of the War)

Biography
Grandfather Paradox founded House Paradox 400 years before the War. Although any member of an Oldblood House was technically allowed to found a new House, the general consensus was that no more Great Houses were needed. When the Grandfather in quick succession founded House Paradox, began talking of alter-time structures, recruited members from other Houses, and established the Eleven-Day Empire, the sluggish Homeworld was simply too shocked to react. They only got around to dealing with the Grandfather after the Imperator crisis forced them to start taking care of their errant members. Knowing that executing the Grandfather would only serve the purposes of his death imagery-obsessed House, the Great Houses simply locked him up on their prison planet.

241 years later, when the Carnival Queen threatened rationality throughout the universe, Lady President Romana released three hundred prisoners, including the Grandfather, during an epileptic fit. (PROSE: Christmas on a Rational Planet) He promptly cut off his arm, removing the Houses' convict tattoo in the symbolic Act of Severance. (PROSE: The Book of the War, Interference) He redefined House Paradox as "Faction" Paradox, no longer protected by the Protocols of the Great Houses and recruiting from the lesser species instead of just Homeworlders. He then "retired" from history, erasing himself from it altogether. (PROSE: The Book of the War)

Gramps
Gramps was a cat living in a retirement home in Florida. He had a dignified manner and appearance, with fur in patterns of dark brown and black that seemed to move and blur impossibly in the shadows, streaks of silver-grey around his face, and no front right leg. He led a group of other cats in a fight against a younger cat — apparently Gramps' past self — in which the younger cat lost a leg. (PROSE: Gramps)

Attack on Gallifrey
A one-armed person claiming to be the Grandfather led an invasion of Gallifrey at the start of the War. The Eighth Doctor initially believed that the man was actually a version of himself from the future, made into a surrogate Grandfather by the Faction (PROSE: The Ancestor Cell) after he had been infected by a Paradox biodata virus during his altered third regeneration on Dust, (PROSE: Interference) but later came to believe that "Grandfather Paradox" appeared to everyone as their most twisted, cynical, even evil future self — the "Ghost of Christmas Cancelled." (PROSE: The Gallifrey Chronicles)

The Faction's invasion of Gallifrey culminated in a confrontation onboard the Edifice between the Eighth Doctor, the Grandfather, and the temporal 'ghost' of the Third Doctor who existed before the Faction changed his history. Faced with the options of leaving Gallifrey or begging the Grandfather for mercy, the Doctor instead chose to fire the Edifice's ancient weapons systems, draining the Edifice of power and forcing the timeline to choose between the Dust and Metebelis III realities, but also destroying Gallifrey in the process.

This Grandfather appeared to the Doctor as 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)

The Grandfather's shadow
Supposedly, the knife which the Grandfather used to cut off his arm was held in the Stacks in the Eleven-Day Empire. The traces of blood it contained were thought to be the only physical remnant of the Grandfather. (PROSE: Act of Severance) More importantly, the knife held the shadow of the Grandfather. Instead of having just one shadow-weapon, the Grandfather's shadow contained an infinite arsenal of every weapon imaginable. (AUDIO: The Eleven Day Empire, The Shadow Play)

Godfather Morlock had it attached to Cousin Justine, who was temporarily shadow-less after following Morlock's instructions to diffuse a Sontaran fusion bomb. The Grandfather's shadow gave Justine access to an infinite arsenal of every weapon imaginable, all contained in her shadow. Shortly afterwards, the Eleven-Day Empire was destroyed and Justine became the Faction's last scion, very literally bearing the Grandfather's spirit or quintessence. (AUDIO: The Eleven Day Empire, The Shadow Play)

Godfather Morlock had previously tried the exact same scheme with Cousin Shuncucker, who also bore the Grandfather's shadow and thought herself "last scion" after the destruction of the Faction's earlier homeworld. Morlock had been forced to abandon the project when it became obvious that Shuncucker was completely insane (a condition perhaps caused by the shadow). (AUDIO: Movers, A Labyrinth of Histories)

Legacy
The Speaker's Seat in the Eleven-Day Empire's Parliament was always left empty to symbolise the Grandfather-who-never-was. (PROSE: The Book of the War)

The Doctor had memories of his mother reading him a legend from a storybook. In the legend, a young man travelled back in time to before his parents were born and murdered his grandfather. The man then became the "Grandfather Paradox" figure, with many more legends associated. He lost an arm — nobody could agree which, or how — and he was alive and dead, murderer and victim, all in one. (PROSE: The Gallifrey Chronicles)

When Cousin Mila ritually summoned spirits on Mercy, the most corporeal spirit appeared as a man missing one of his arms. Which arm was impossible to make out. (PROSE: Holding Pattern)

Aki knew she would have to become the Grandfather's hand to save the bone people from the witch-woman. The babel said that it was a more powerful shadow than any the Grandfather cast. (PROSE: Newtons Sleep)

Academician Devonire became obsessed with finding the Grandfather's severed arm and presenting it to Faction Paradox as a peace offering from the Great Houses. He believed that the Immaculata Formosii had the arm, and at a meeting on Kaiwar, Formosii promised to give it to him in exchange for his own arm. Devonire rejected this offer, stole the arm, and escaped. However, the arm he stole was in fact his own; this sparked severe paradox anxiety in Devonire, who eventually cut off the matching arm, threw it off his balcony, and declared himself to be Faction Paradox, saying that the Faction had created him just as the Grandfather had created the Faction. The Houses froze him in their prison, partially in fear that he was right. (PROSE: The Book of the War)

The Remote treated the word "Grandfather" as a vile obscenity. (PROSE: The Taking of Planet 5)

Behind the scenes

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