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The Yssgaroth (also called Great Vampires) were a force of destruction unleashed into the Spiral Politic during the Dark Times and fought by the Time Lords.

According to The Book of the War, it was unclear whether they were a species or a side effect of the collision of the Spiral Politic with their universe, and whether there were multiple Yssgaroth, or if there was just one, capable of splitting itself into smaller forms. (PROSE: The Book of the War)

Biology
The Yssgaroth could manifest in multiple forms, varying based on what species or individual was perceiving them. These included stems of bloody thorns, tearing flesh from whole worlds; bleeding, eyeless lambs on divine thrones; (PROSE: The Book of the War) horrible, huge, winged snakes with many eyes; (PROSE: The Pit and blind, screaming abominations made of leather wings and shredded muscle. (PROSE: The Book of the War, TV: State of Decay)

Like lesser vampires, Great Vampires needed to steal energy from other sources to survive. Just one was extremely powerful and could drain a whole planet of energy. (TV: State of Decay)

The Yssgaroth also had extreme regenerative abilities, and they could survive numerous wounds. (PROSE: The Book of the War) Leela's vision was restored after she drank Great Vampire blood. (PROSE: Annihilation) The only way to kill one was to stab them through the heart, usually with a steel bolt. More powerful Great Vampires needed to have their heart fully destroyed in order to kill them. (TV: State of Decay)

Yssgaroth biomass was able to hybridise and corrupt ordinary biomass by contact. Richard Francis Burton hypothesized that the Great Houses obtained the ability of regeneration by intentionally modifying themselves with the Yssgaroth "taint". (PROSE: The Book of the War)

History
Because the story of the Yssgaroth was never recorded in the Matrix, there were varying accounts of their origin; however, whether in one of Rassilon's early experiments creating black holes (PROSE: The Pit, Interference) or during the anchoring of the thread, (PROSE: The Book of the War) all accounts agreed that they were accidentally released from their dimension by the Time Lords (AUDIO: Project: Twilight) during the Dark Times. (TV: The Infinite Quest)

Through the Eternal War, the Time Lords were able to drive the Yssgaroth back into their own universe; (PROSE: The Pit) the only survivor was the swarm leader, (PROSE: Interference who escaped into E-Space and was later killed by the Fourth Doctor. (TV: State of Decay) However, since they had emerged at the beginning of history, the Yssgaroth taint remained in the fabric of the universe and couldn't be removed without the unweaving of time itself. On Earth, the Yssgaroth taint survived in the Mal'akh (PROSE: The Book of the War) and the vampires (PROSE: Vampire Science)

Some Yssgaroth survived and tried to get back into the normal universe. (PROSE: The Pit)

One of Rassilon's titles was "Conqueror of Yssgaroth". (AUDIO: Zagreus) His throne appeared to be made of the skeletons of giant bats. (PROSE: Christmas on a Rational Planet)

In a TARDIS projection during the Eighth Doctor's infection with anti-time, the head of the Arcalian Chapter Houses, Provost Lord Tepesh, revealed himself as a Great Vampire in disguise, even saying he was a direct descendant of the King Vampire. Tepesh claimed the Great Vampires were a peaceful lot, content to drink the blood of non-sentient, genetically bred cattle, until Rassilon showed up, declared the Vampires should not be allowed to exist, and began a 500-year war of extermination. (AUDIO: Zagreus)

Alternate timeline
During one of Romana II's, Leela's and Narvin's trips to alternative Gallifreys they visited one where Rassilon, after completing the transduction barrier, was killed by the Great Vampires, who then darkened the barrier to block out all light so they might live safely on Gallifrey. Over the next couple of millennia the vampires gradually took over the True Lords, until the battle between the vampire Lord Prydon and the True Lady Magestrix Borusa. During this battle, with Leela's help, Narvin and Romana helped Borusa and recaptured the Transduction barrier, increasing the light levels allowed to pass through and destroying the Vampires. However, this made Romana feel like she was becoming a "Destroyer of Worlds" as according to her she had killed the Vampires and the Time Lords of her universe. (AUDIO: Annihilation)

Behind the scenes

 * According to the invalid source Doctor Who: Creatures and Demons, the Plasmavores were descended from the Great Vampires.