Information from the Faction Paradox series needs to be added.
Sabbath was born in 1740. He studied at Cambridge and worked for the Security Service. (PROSE: The Adventuress of Henrietta Street) He was initiated into the Service in 1762 at the age of twenty-one. Sabbath was not supposed to have survived his initiation, but was saved by Octan and the Council of Eight to be used as a counterpoint to the Eighth Doctor. (PROSE: Sometime Never...) Sabbath defected from the Secret Service in 1780. (PROSE: The Adventuress of Henrietta Street) Shortly after this, Sabbath was contacted by the Council of Eight after a period of almost twenty years since his last encounter with them. The Council of Eight approached Sabbath at St. Cedd's College, and warned him of the impending arrival of the Doctor. (PROSE: Sometime Never...)
Sabbath encountered "Leviathan" under the River Thames as part of his indoctrination ceremony into the Service. He set up his own power base independent of the Service. After leaving the Service, Sabbath sent a message via a number of corpses of the Service's "ratcatchers" saying, "leave me alone, and I'll leave you alone".
The Doctor chose Sabbath for his best man at his wedding to Scarlette. Sabbath later removed the Doctor's second heart, allegedly to cure the Doctor of an illness caused by the heart's attempt to connect the Doctor to the non-existent Gallifrey, but also allowing Sabbath to travel through time in the Jonah, endowing him with the Doctor's ability to penetrate "deep time" — exploring the universe beyond his home era and planet — that would have been impossible in his more primitive ship otherwise. (PROSE: The Adventuress of Henrietta Street)
Sabbath had agents which were to him the same as companions were to the Doctor. Among them were Juliette Vierge, (PROSE:The Adventuress of Henrietta Street) Sasha and Jueves, (PROSE: History 101) Jaxa and Roja, former Time Agents, (PROSE: Trading Futures) and The Angel Maker (aka Elizabeth Kelly). (PROSE: Camera Obscura)
Sabbath arranged for the Doctor and the Clock People to meet on a Plutocratic Empire colony planet, manipulating the Doctor's perception of the situation so that he would see the creatures as a threat rather than evacuees. (PROSE: Anachrophobia) He later tricked the Doctor into destroying the System, a mysterious information-gathering program that allegedly contained information about his initiation that should not be made public. (PROSE: History 101)
While near death, Sabbath met Death, learning that the Doctor's second heart created a link between the Doctor and Sabbath that prevented the Doctor from dying; even if his chest was virtually crushed, one of his hearts was still beating in Sabbath. After this link resulted in the Angel-Maker sacrificing herself to save the Doctor in the belief that she would also save Sabbath, Sabbath tore out the heart, allowing the Doctor to grow a new one. (PROSE: Camera Obscura)
Sabbath was sent by his masters to take actions that would allegedly "collapse" reality into a single manageable timeline, convinced that time-travellers weakened reality by creating new universes every time they went back in time. This plan failed, and instead Sabbath's actions caused every parallel universe to overlap and fight for dominance. (PROSE: Time Zero) Realising that his plans had not worked, Sabbath kidnapped two of the Warlocks of Demigest and planned to use their powers to his advantage. He took the Warlocks in the Jonah to Selonart, where he attempted to become one with the universe due to the unique conditions on the planet. He planned to use the Warlocks to then help him control the universe. Sabbath's plan backfired due to the intervention of the Doctor and Bloom. Sabbath was then taken by the Warlocks back to Demigest where he was kept as their prisoner. The Doctor arrived later and killed the Warlocks, allowing Sabbath to escape. (PROSE: The Infinity Race) Sabbath later assisted Fitz Kreiner, Anji Kapoor and the Doctor when the multiple realities began to converge and mix. (PROSE: The Last Resort)
Sabbath was finally killed after turning his back on his employers and helping the Doctor; realising that his employers gained power from predicting future events, and with their plans now dependent on Sabbath killing either the Doctor or the head of the Council, Sabbath instead shot himself, something that the Council "knew" he wouldn't do. (PROSE: Sometime Never...)
In an alternate timeline, a version of Sabbath imprisoned Alan Turing, preventing the creation of the computer. He believed that his actions were to turn Turing into a temporal nexus whose death would cut Sabbath's world off from the rest of the multiverse and protect it from the catastrophe that had been unintentionally set off by the "prime" Sabbath, but in reality killing Turing just accelerated the multiversal damage. (PROSE: The Domino Effect)
Sabbath was generally opposed to chaos and believed that unauthorised time travel created parallel universes which threatened reality. Sabbath was confident in his mental abilities. Aided by the Doctor's second heart, he survived a journey into Death's realms. (PROSE: Camera Obscura)
He was incredibly stubborn and disliked beings who lorded their powers over others, including the Doctor. However, this arrogance also meant that he found it hard to admit when he was wrong, constantly convinced that he understood time better than the Doctor even after the Doctor provided evidence that time travel did not work the way Sabbath believed it did. (PROSE: Time Zero)
Sabbath used many pseudonyms, often variations upon his chosen name of Sabbath, such as Holiday or Mistletoe. (PROSE: Anachrophobia, Time Zero) Although he openly scorned his agents, he was fiercely loyal to them. (PROSE: The Adventuress of Henrietta Street)
Sabbath was bulky and muscular. His body was getting fat, having concluded that he was better served as a behind-the-scenes manipulator rather than taking action himself. He kept his head almost shaved. This was not a political statement or an attempt to intimidate people, but to save time and energy in grooming. (PROSE: The Adventuress of Henrietta Street) Many of Sabbath's clothing choices echoed the Doctor's; for example, he wore a linen suit after adopting the role of Time's Champion. (PROSE: Camera Obscura)