Great Vampire

The Yssgaroth were horrible, huge, winged snakes with many eyes, but this may be illusion, since their appearance causes hallucination. They were native to an another universe, but they wanted to be released into the normal universe. (NA: The Pit)

They may be one and the same as the Great Vampires. "Pure Yssgaroth" (ie not beings infected by Yssgaroth "blood") are more a form of psychotic space then a living being, they're sentient and masters of torture, capable of "spreading your nervous system across continents and keeping you unconscious to everything except pain". Yssgaroth "blood" is infectious and exposure creates beings dedicated to corruption, destruction and pain. It is possible that they could even infect a Time Ship, one such Time Ship is Lolita. (FP:The Book of the War)

The Yssgaroths and their universe were discovered by the Time Lords in one of Rassilon's early attempts to create a black hole. Instead of creating a black hole, they accidentally punched a hole through space 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. Even then, 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)