Slythell are rumored to be rot-covered worms that have congealed into a writhing mess about the size of a child. They live underground, only come out at night, and no one knows how (or where) they were made. When they get too close to a water system, the water is poisoned and subsequently the attached population is sickened with fever, weakness, rash, vomiting, diarrhea, and bloody blisters in their mouths. Medicine and magic combined together can cure the slyth, but it will reinfect if the slythell isn't found and eradicated.
Some tall (presumably) tales say that slythell cannot be killed, only relocated. They rot even weapons and clothing they come into contact with. They say they have beady eyes under the writhing mess but no visible arms, legs, or mouth. Despite being a blobbish creature, they spit and pop their writhing goo onto the surrounding areas (and persons if near enough). When disturbed, it is rumored that they will lash out and attack. Any touched by their rot-covered exterior gets sick with bloody blisters at the point of contact...