Monstrous Gravedigger / Possessed Victim
Mourngrym is an unassuming gravedigger in Shadowdale. But the adventurers have exposed him to be a monstrous, perhaps undead fiend with magical powers. He also seems to command rats to help him.
He has fled the first encounter with the party by teleporting away in a flash of green light.
As it has turned out, Mourngrym has been possessed by a guecubu, an abyssal demonic entity. He has been slain by the adventurers, but his origin remains mysterious.