The rot worm is said to be a slug-like blob, with spiny roots that can reach into outer space, if required. It's said to have a mouth like a leech on top, capable of consuming an adult Pendragon in a single "bite".
It's nothing that proves that this monstrosity of a plant really exists in the dark confines of the earth, but it has been many sightings of spiny tentacles growing from the planets surface. These bizarre things is said to be the rot worm's roots, and that they gather nutrition for the rot worm by rending and tearing things to beyond recognition.