§ Of Beren and Lúthien The Silmarillion

There dismay took them, for at the gate was a guard of whom no tidings had yet gone forth. Rumour of he knew not what designs abroad among the princes of the Elves had come to Morgoth, and ever down the aisles of the forest was heard the baying of Huan, the great hound of war, whom long ago the Valar unleashed. Then Morgoth recalled the doom of Huan, and he chose one from among the whelps of the race of Draugluin; and he fed him with his own hand upon living flesh, and put his power upon him. Swiftly the wolf grew, until he could creep into no den, but lay huge and hungry before the feet of Morgoth. There the fire and anguish of hell entered into him, and he became filled with a devouring spirit, tormented, terrible, and strong. Carcharoth, the Red Maw, he is named in the tales of those days, and Anfauglir, the Jaws of Thirst. And Morgoth set him to lie unsleeping before the doors of Angband, lest Huan come.