Rotworms

Lore
Born of the decaying detritus which carpets the swamplands, these loathsome invertebrates are more nuisance than threat--a pass of your torch across their path will usually keep them at bay.

- from "Compendium", Ultima VI

''In the dank depths of the Underworld - where rot infects, corrosion ruins and corruption festers - can be found the foul nests of these loathsome worms. Should one approach close enough to be sensed by a Rotworm, the beast is sure to attack. An armored warrior has little to fear from these worms, but an ill-equipped explorer could easily be overcome by sheer numbers.''

- from "The Memoirs of Cabirus", Ultima Underworld

''The rotworm, cousin to the bloodworm, feeds on decaying organic matter, and thus has no shortage of food in the underworld. These worms will always attack when they sense flesh nearby, but are easily crushed to  death with a well-aimed kick. The only danger lies in being overwhelmed by a large number of rotworms clustered around a carcass.''

- from "A Safe Passage through Britannia", Ultima Underworld II

Evolution in the series
First appearance: Ultima V

Last appearance: Ultima Underworld II

They are the same in all installments in which they appear, but stronger in both Underworlds.

In both Underworlds, the Avatar also encountered the far more dangerous Bloodworms, a offshoot of Rotworms.

Trivia

 * In Ultima Underworld I the Avatar must use the corpse of a rotworm as an ingredient to 'Wormy Stew' for a troll to progress in the game.