The Prose Edda 1.12

Then said Gangleri: "The sun fares swiftly, and almost as if she were afraid: she could not hasten her course any the more if she feared her destruction." Then Hárr made answer: "It is no marvel that she hastens furiously: close cometh he that seeks her, and she has no escape save to run away." Then said Gangleri: "Who is he that causes her this disquiet?" Hárr replied: "It is two wolves; and he that runs after her is called Skoll; she fears him, and he shall take her. But he that leaps before her is called Hati Hródvitnisson. He is eager to seize the moon; and so it must be." Then said Gangleri: "What is the race of the wolves?" Hárr answered: "A witch dwells to the east of Midgard, in the forest called Ironwood: in that wood dwell the troll-women, who are known as Ironwood-Women. The old witch bears many giants for sons, and all in the shape of wolves; and from this source are these wolves sprung. The saying runs thus: from this race shall come one that shall be mightiest of all, he that is named Moon-Hound; he shall be filled with the flesh of all those men that die, and he shall swallow the moon, and sprinkle with blood the heavens and all the lair; thereof shall the sun lose her shining, and the winds in that day shall be unquiet and roar on every side. So it says in Völuspá:

Eastward dwells the Old One in Ironwood, And there gives birth to Fenrir's brethren; There shall spring of them all a certain one, The moon's taker in troll's likeness.

He is filled with flesh of fey men. Reddens the gods' seats with ruddy blood-gouts; Swart becomes sunshine in summers after, The weather all shifty. Wit ye yet, or what?"