Yssgaroth

The Yssgaroth are horrible creatures mentioned in The Pit. They are huge, winged snakes with many eyes, but this may be illusion, since their appearance can cause hallucination. They are native to an alternate dimension, but they want to be released into the normal universe.

The Yssgaroths and their alternate dimension were discovered by the Time Lords in one of Rassilon's early attempt to create black holes. Instead of creating a black hole, they accidentally punched a whole through space into the alternate dimension. 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. 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.