Spiral Yssgaroth

The Spiral Yssgaroth was a name given by The Book of the War to the universe from which the Yssgaroth, a powerful force of darkness not bound by the structures and laws of the Great Houses' Spiral Politic, originated. During early Gallifreyan history, the Time Lords accidentally opened a gateway between their universe and the Spiral Yssgaroth, allowing the Great Vampires to swarm into the cosmos.

After they banished the Vampires back to their universe, the Yssgaroth continued to escape back into their universe to conquer it and get their revenge; their realm, connected to Earth at a number of weak points or "leprous houses", was thought of as a reservoir of evil, river of evil or reservoir of pain by Doctor Colin Dove and Director Giles prior to Giles learning the true nature of the realm.

Nature
The Book of the War described the Spiral Yssgaroth as an alter-matter state whose structures and protocols were completely hostile to those of the Spiral Politic. It further speculated that the Yssgaroth entity or entities were simply side-effects of the collision between the Spiral Yssgaroth and the Spiral Politic, and the Yssgaroth were simply the Great Houses and lesser species' projections of their fears onto areas of hostile anti-structure. (PROSE: The Book of the War) Other sources, however, instead speculated that the warped Yssgaroth creatures who crossed over into the Great Houses were mutated versions of the parallel world's equivalents of the Great Houses. (PROSE: The Cosmology of the Spiral Politic)

The terrain appeared to be mostly desert. The universe had unusual proportions and distances, making travel even more difficult. There were many settlements of suffering humans, who were generally used as slave labour. It was the origin of Dream B. Species including Humans, Ogrons, Terileptils, the Galk, and Bapputchin had been accidentally brought to the Yssgaroth's universe. Other species, such as the Cun, were either native or just unknown to the Seventh Doctor. Still others, like the Hunters, could naturally travel to the Yssgaroth's universe. (PROSE: The Pit)

The universe of Earth and the "reservoir of pain and evil" that was the Yssgaroth's universe were "close" to each other within the space between dimensions, (PROSE: Preternatural Nights) becoming closest during Perihelion events. There were "synapses", traditionally called leprous houses, between the two universes; sensitives who ventured too close to these points of connection, especially near in time to a Perihelion event, could become directly possessed by the "evil force" on the other side. (HOMEVID: The Zero Imperative) After falling out of the universe into the space between dimensions, the human Archie MacTavish found himself "facing" the "darkness" of the Yssgaroth's universe, and the Yssgaroth intelligence was able to reach out and communicate to him after noticing his presence, helping him reenter his native universe after making him its agent. (PROSE: Preternatural Nights)

History
The Time Lords originally discovered the Yssgaroth's universe during early experiments in making black holes. These experiments accidentally created ruptures in the space-time continuum, releasing the Yssgaroth. (PROSE: The Pit) One such opening was on the Homeworld itself; (PROSE: The Book of the War) Other openings to the Yssgaroth's universe could be found in 19th century London, near Stonehenge, and on the unnamed planet in the Althosian system. (PROSE: The Pit)

On 13 August, 1945, the force possessed young Daniel O'Kane and had him murder his mother, father and sisters. Daniel was studied by Doctor Colin Dove, who became obsessed with understanding the "reservoir of evil" Daniel seemed to tap into, and what would happen upon a successful Perihelion event. By August 1994, the force had killed nine people through Daniel before killing William Bruffin, whom it reanimated briefly. Doctor Dove saw this as a sign that it was almost perihelion and time for Daniel and Russell to meet. The force killed Doctor Dove when he was no longer of use and was breaking through before Daniel, in a moment of clarity, jumped in front of Russell's blade and saved Dr Liz Shaw. Daniel's death meant that the force could not be unleashed. The force still had a connection to the physical world, however, as shown through a psychic connection between Russell and Patient One. (HOMEVID: The Zero Imperative)

During the War in Heaven, according to Auteur's writings, Lolita pressed against the Very Fabric of Time and Space to give Dracula a glimpse of the Spiral Yssgaroth. (PROSE: A Bloody (And Public) Domaine) During the Last Great Time War, the Time Lords, at the command of Cardinal Ollistra, released the Great Vampires from their universe to fight off a fleet of Dalek saucers. Despite the War Doctor's protests, Councilor Voltrix proceeded to open a rift, allowing several to break through. (COMIC: The Bidding War)

After being cast out of his home universe in 2018, Archie MacTavish made contact with the Yssgaroth's universe while floating, frozen in time, in the space between dimensions. He made a deal with the Yssgaroth entity, which helped him return to his universe to work on a ritual in time for the next Perihelion that would open a full gateway between the two universes. On the night of the ritual, it began to break through, the Yssgaroth mentally transported Azacca Dixon to its universe to break his will scare him into submission, but was unsuccessful. Ultimately, P.R.O.B.E. successfully interrupted Archie's ritual, closing the gateway and preventing the two universes from fully colliding. (PROSE: Preternatural Nights)

Behind the scenes
The P.R.O.B.E. film The Zero Imperative did not originally feature any reference to the Yssgaroth, or otherwise tie into the mythos of State of Decay or The Pit. Its highly ambiguous "reservoir of pain" from beyond the physical universe, and the evil force it contained, were only established as synonymous with the Yssgaroth and their universe in the 2021 short story anthology P.R.O.B.E.: Out of the Shadows.