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. (NA: The Pit)
They may have been one and the same as the Great Vampires. "Pure Yssgaroth" (ie not beings infected by Yssgaroth "blood") were more a form of psychotic space then a living being, sentient and masters of torture, capable of "spreading your nervous system across continents and keeping you unconscious to everything except pain". Yssgaroth "blood" was infectious and exposure created beings dedicated to corruption, destruction and pain. It is possible that the Yssgaroth could infect a Time Ship. One such Time Ship was 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, 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)