She served as a herald, or a messenger, for Sauron at Tol-Sirion.
"...she was wont to fly in vampire's form to Angband; and her great fingered wings were barbed at each joint's end with an iron claw." The SIlmarillion, chapter 19: Of Beren and Luthien. page 178, 4th paragraph
Additionally, Luthien disguised herself as Thuringwethil in order to infiltrate Angband. She cast off the disguise, however, at the entrance to Angband.
Thuringwethil was a Sindarin word that meant 'Woman of Secret Shadow'.