Great Vampire

The Yssgaroth were horrible, huge, winged snakes with many eyes, but this may have been illusion, since their appearance caused hallucination. They were native to another universe, but they wanted to enter the normal universe.

The Yssgaroths 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). Though they were able to push the Yssgaroths back to their own universe, billions of people were killed and whole star systems were destroyed. The Yssgaroth survived and tried to get back into the normal universe. (NA: The Pit)

One of Rassilon's titles was "Conqueror of Yssgaroth". (BFA: Zagreus)

Behind the scenes
The backstory of the Yssgaroth is similar to that of the Great Vampires, and in Lawrence Miles' Faction Paradox universe, the two seem to be combined.