The Great Vampires (also called the Yssgaroth or the djinn) were a force of destruction in Gallifreyan legend that were unleashed into the universe during the Dark Times and fought by the Time Lords.
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)
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. (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. This property was called the Yssgaroth "taint". According to The Book of the War, it was unclear whether the Yssgaroth 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)
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)
Throughout the Dark Times, (TV: The Infinite Quest) the Great 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) Earth's vampires, (PROSE: Vampire Science) and the Saturnyns. (PROSE: The Multi-Faceted War) 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)
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 Great 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 forced-matter shells disguised as planets, including Earth. (PROSE: Interference)
Though the Time Lords were able to find and destroy all other Vampires, 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)
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 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)
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. 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 Doctor Who: Creatures and Demons, a source this wiki considers invalid, the Plasmavores are believed to be descended from the Great Vampires.