Great Vampire

The Great Vampires, or Yssgaroth, were a force of destruction in Gallifreyan legend and history that were unleashed into the universe during the Dark Times and fought by the Time Lords in the Eternal 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) huge winged snakes with horns, many eyes, and wet, dripping skin; (PROSE: The Pit) clouds of smoke that would fill a planet's atmosphere and induce nuclear winter; (PROSE: Predating the Predators) and blind, screaming abominations made of leather wings and shredded muscle. (TV: State of Decay, PROSE: The Book of the War, Predating the Predators)

It was often speculated that there was only one Yssgaroth, capable of splitting itself into smaller forms. Some believed that, rather than being alive, the Yssgaroth were areas of hostile anti-structure produced by the collision of the Spiral Politic with another universe, turned into monsters by the Great Houses' and lesser species' projections of their fears. (PROSE: The Book of the War) Another view was that the monstrosities were actually members of counterpart Great Houses from that other universe. (PROSE: The Cosmology of the Spiral Politic)

The 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) With their incredible hypnotic powers, the Vampires could enslave entire planetary populations. (PROSE: Predating the Predators)

They 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. (AUDIO: Annihilation) The only way to kill one was to stab them through the heart, usually with a steel bolt. More powerful 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. This property was called the Yssgaroth "taint" (PROSE: The Book of the War) and was caused by the application of the Yssgaroth observer effect on the Spiral Politic. (PROSE: The Cosmology of the Spiral Politic)

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) At that instant, one Yssgaroth appeared on the Homeworld, at the site that would later be known as the caldera, as thousands of "servants of the Yssgaroth" began to tear their way into the continuum at many points throughout history. (PROSE: The Book of the War)

Throughout the Dark Times, (TV: The Infinite Quest) the Vampires caused mass destruction and chaos throughout the cosmos. (TV: State of Decay) Through their taint, they created the Mal'akh, (PROSE: The Book of the War) the Saturnyns, (PROSE: The Multi-Faceted War) and Earth's vampires. (PROSE: Vampire Science) The First Doctor also speculated that the Sou(ou)shi were related to the Great Vampires. (PROSE: Venusian Lullaby)

Richard Francis Burton hypothesised that the Great Houses obtained the ability of regeneration by intentionally modifying themselves with the Yssgaroth "taint". (PROSE: The Book of the War) One of the ROO texts revered by the Cult of Rassilon the Vampire recorded that Rassilon allowed himself to be bitten by a Great Vampire and was a vampire himself at the end of his life. (PROSE: Goth Opera)

The ferocity and resilience of the Vampires made their defeat very difficult, even for the Time Lords. (TV: State of Decay) They waged the lengthy Eternal War to drive them back. (PROSE: The Pit) Since conventional weapons were virtually useless, Rassilon ordered the construction of bowships to exploit the one weakness of the Vampires — their vulnerability to being impaled through the heart. (TV: State of Decay) Though billions of people were killed and whole star systems were destroyed, (PROSE: The Pit) the Time Lords were finally able to push the Yssgaroth back into their own universe, and the outlets to their universe were blocked and reinforced by containment shells disguised as planets, including Earth. (PROSE: Interference)

During the war, entire worlds were turned into laboratories of cruelty where the Yssgaroth destroyed the bodies of the inhabitants until only their nervous systems remained, alive and in agony but without any hope of help. (PROSE: The Book of the War) Every thirteen local years, their servants on each world would round up a thirteenth of the population and ship them to the Vampires' galactic headquarters in the Blood Citadel of Alukah, where they would be harvested for blood. (PROSE: Predating the Predators) These "fallen" worlds were retro-annulled by the Great Houses at the end of the war. (PROSE: The Book of the War)

Though the Time Lords were able to find and destroy all other Yssgaroth, the swarm leader was able to escape. (PROSE: Interference) This sole survivor hid in E-Space, using his great power to draw the ship Hydrax with him. He turned the three humans on this ship into vampires to feed and serve him for a thousand years until the Time of Arising. As he rose from the ground, the Fourth Doctor used a scout ship from the Hydrax to pierce his heart, killing him. (TV: State of Decay)

Since they had emerged at the beginning of history, the Yssgaroth taint remained in the fabric of the universe even after their defeat; it couldn't be removed without unweaving time itself. (PROSE: The Book of the War)

Some Yssgaroth survived and tried to reenter the 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)

When Professor Stahlman began the Inferno Project to drill into Earth's crust, the released ooze transformed exposed humans into Primords; (TV: Inferno) a similar effect was observed among the Wyrm Callers when the Wyrm was released after something impacted Earth and destroyed the dinosaurs. (AUDIO: The Adolescence of Time) After plugging the last hole to the Yssgaroth's universe with an artificial planet, the Engineer murmured to himself that he hoped no one would ever try drilling through to the core. (PROSE: Interference)

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 them through. The memories of this event proved the Ninth Doctor's memories of the Time War were authentic. (COMIC: The Bidding War)

The Cold could suck its target into another universe, where it would be destroyed by the inhabitants. Those creatures sought to enter the universe through the Cold, attacking it with their gigantic black wings, enormous clawed hands, and huge teeth. One's face was the size of a Drashig. (PROSE: Interference)

Lolita once pressed against the Very Fabric of Time and Space to show Count Dracula a glimpse of the Yssgaroth wanting to enter the Spiral Politic. (PROSE: A Blooy (And Public) Domaine)

Jendrickenses speculated that the Yssgaroth would be able to return through the Grandfather's Maw. (PROSE: The Brakespeare Voyage)

An alternate perspective
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 was killed by the Great Vampires after completing the transduction barrier. The Vampires then darkened the barrier to block out all light so they might live safely on Gallifrey. Over the next 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. (AUDIO: Annihilation)

Behind the scenes

 * According to Doctor Who: Creatures and Demons, the Plasmavores were descended from the Great Vampires. However, that source is considered invalid by this wiki.
 * About the use of the Yssgaroth in the Faction Paradox series, Lawrence Miles said, "The idea of a formless horror from before the dawn of time is generic, but 'Yssgaroth' is the best name I've ever heard, so I thought it'd be better to get Neil's permission than to use a completely different word for the same thing." (REF: Bernice Summerfield: The Inside Story)