Fenrir is one of the three born to Loki and the giantess Angrboða; the others are Jörmungandr, the serpent that circles the world, and Hel, who keeps the place of the dead. The gods raised him in Asgard, and he grew so fast that only Týr would go near him.
Hearing the prophecy that he would swallow Odin at Ragnarök, they decided to bind him. Twice he broke the chains. The third binding was Gleipnir, made by dwarves and woven, it is said, from a cat’s footfall, a woman’s beard, the roots of a mountain, a bear’s sinews, a fish’s breath, and a bird’s spittle — made of things that do not exist, and so unbreakable by anything that does. Suspecting a trick, Fenrir agreed to be bound only if one of the gods would put a hand in his mouth. Týr put in his hand, and when the ribbon held, Fenrir took it.
The story lasts because its ending is postponed rather than changed. The gods could not kill him. They bound him on the island of Lyngvi, paid a hand for the binding, and the day he comes loose is already written down. They bound him because they had heard the prophecy, and because they bound him, Odin will be standing there when he is free.