Kazgaroth

Kazgoroth is a cursed demon that takes the form of anything it wants to and drinks human blood, as well as feasts on human flesh. It takes down many normally powerful people, like a druid, in the book Darkwalker on Moonshae.

Guises
Kazgoroth, to fit in, has taken several guises. The first few are a cow, which attracted a bit too much attention, and a woman, who drew much too much attention because Kazgoroth used its powers to make the body of the latter guise more attracive. His first victim was a man named Erian who was a guard for the prince Tristan of the Moonshaes.

Trivia

 * Kazgoroth is the first villain ever encountered in Forgotten Realms.
 * This demon was a creation of Douglas Niles, the first Forgotten Realms author ever.
 * This villain, as well as Darkwalker on Moonshae itself according to some fans, is the reason Forgotten Realms got so popular as a Dungeons and Dragons campaign setting.