§ Of Beren and Lúthien The Silmarillion

But Lúthien heard his answering voice, and she sang then a song of greater power. The wolves howled, and the isle trembled. Sauron stood in the high tower, wrapped in his black thought ;but he smiled hearing her voice, for he knew that it was the daughter of Melian. The fame of the beauty of Lúthien and the wonder of her song had long gone forth from Doriath; and he thought to make her captive and hand her over to the power of Morgoth, for his reward would be great. Therefore he sent a wolf to the bridge. But Huan slew it silently. Still Sauron sent others one by one; and one by one Huan took them by the throat and slew them. Then Sauron sent Draugluin, a dread beast, old in evil lord and sire of the werewolves of Angband. His might was great; and the battle of Huan and Draugluin was long and fierce. Yet at length Draugluin escaped, and fleeing back into the tower he died before Sauron's feet; and as he died he told his master: 'Huan is there!' Now Sauron knew well, as did all in that land, the fate that was decreed for the hound of Valinor, and it came into his thought that he himself would accomplish it. Therefore he took upon himself the form of a werewolf, and made himself the mightiest that had yet walked the world; and he came forth to win the passage of the bridge.