Great Vampire

The Yssgaroth were a force of destruction unleashed onto the Spiral Politic during the anchoring of the thread. 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)

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)

The Yssgaroth and their universe were discovered by the Time Lords in one of Rassilon's early attempts to create a black hole. Instead, he accidentally punched a hole into Yssgaroth's universe. The Yssgaroth escaped and caused mass destruction. The Time Lords, led by Kopyion Liall a Mahajetsu, tried to stop them, leading to the thousand year Eternal War (also known as the First Great Time War or the First War in Heaven). Though billions of people were killed and whole star systems were destroyed, the Time Lords were able to push the Yssgaroth back into their own universe, (PROSE: The Pit) reinforcing node points of the metastructure of history with forced-matter shells and disguising them as planets. (PROSE: The Book of the War) Earth was one of these planets. (PROSE: Interference, Head of State)

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)

Behind the scenes
The backstory of the Yssgaroth is similar to that of the Great Vampires; in PROSE: Interference and PROSE: The Book of the War, the two are confirmed to be the same.