Wormy was an alleged lieutenant of Cornello who ran a cult of disgruntled Verandi refugees in the Vistrian capital of Castle. By using trained scarabs, he stole blood from Vistrian children and attempted to use it to build a portal to the Shadowfel. Zaheer and Mahar discovered this and, with the aid of the rest of the party, killed Wormy and broke up his cult.
Although at first he appeared merely to be a portly man with a charasmatic, if somewhat demagoguic, preaching style, there was more to Wormy than met the eye. His human form was in fact a mere "suit" created from a human corpse, inside which he magically compressed his true form - a giant worm.
As well as the natural advantages that come from being a giant worm (strength, crushing damage, being terrifying), he had a maw full of negative energy (which dealt continuous damage to anybody he swallowed) and could regenerate from any damage except positive energy. This implies that he was in fact some sort of undead.
Questions over true loyalties
According the the Rahjs, Wormy is ordinarily a servant of Swayne. Furthermore, the area where he was trying to open a portal, Castle, is in the heart of Swayne's territory, so he would be the one to gain the souls/bodies/whatever other resources travelled through the portal. These facts combined shed serious doubt on whether Wormy truly served Cornello, although it is unclear as to why he would lie to adventurers he fully expected to kill, or cultists who likely knew nothing of the Vampire Kings in the first place.
Furthermore, in the after-credits scene of Episode 54, Voyvoda informs Cornello that Wendigo had failed to retake "the worm shrine" from Swayne and that it cannot be retaken unless "they" help again. Assuming that "the worm shrine" is the sprit shackle responsible for Wormy's powers, this would imply that Wormy was briefly one of Cornello's lieutenants.