"Thuringwethil was a vampire, capable of assuming the form of a great bat, who served as the messenger of Sauron, during the First Age. She likely also served Morgoth.
The origins of Thuringwethil are untold. Her ability to change form suggests that she may have been an Ainu or a skin-changer. In the form of a vampire, she had at least two great, dark fingered wings with an iron claw at the end of each joint.
Sometime before the Quest for the Silmaril, Thuringwethil became the messenger of Sauron at Tol-in-Gaurhoth, formerly Tol Sirion, often flying to Angband in the form of a great bat, to bear messages.
She was there slain by Huan, during the Quest for the Silmaril, along with Draugluin, when Sauron sent out his army one-by-one." Courtesy of The One Ring Wiki