Reluctant to forsake his body

But no wizardry nor spell, neither fang nor venom, nor devil’s art nor beaststrength, could overthrow Huan of Valinor; and he took his foe by the throat and pinned him down. Then Sauron shifted shape, from wolf to serpent, and from monster to his own accustomed form; but he could not elude the grip of Huan without forsaking his body utterly. Ere his foul spirit left its dark house, Lúthien came to him, and said that he should be stripped of his raiment of flesh, and his ghost be sent quaking back to Morgoth; and she said: ‘There everlastingly thy naked self shall endure the torment of his scorn, pierced by his eyes, unless thou yield to me the mastery of thy tower.’

Then Sauron yielded himself, and Lúthien took the mastery of the isle and all that was there; and Huan released him. And immediately he took the form of a vampire, great as a dark cloud across the moon, and he fled, dripping blood from his throat upon the trees, and came to Taur-nu-Fuin, and dwelt there, filling it with horror.

-The Silmarillion