Luthien transforms herself into Thuringwethil, Sindarin for 'woman of secret shadow'. She was a messenger of Saurons in the first age who i believe only took the form of a giant vampire bat, not actually being a vampire. Perhaps this ability to transform also indicates that she too was a 'corrupted' Maiar or another enigma or anomoly of the story. I just love how Tolkien plants them everywhere throughout his stories so we can let our imaginations run free!
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for Sauron took to himself the name of Annatar; the Lord of Gifts
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