According to the myths, the evil Loki himself gave birth to Fenrir, after eating the heart of a giantess, the witch Angrboda. After his birth, the gods received prophecies of disaster concerning Fenrir and his siblings. Even when Fenrir was a pup, the only god courageous enough to approach him was Tyr.
Despite the dire prophecies, according to the 'Prose (or Younger) Edda', the gods could not kill Fenrir because it would have defiled their sanctuary. But they sought some way to tie up the beast, who grew noticeably larger each day. They attempted to restrain him with two different iron fetters. The wolf broke the first, called Leyding, with a single kick. The second fetter, called Dromi, was twice as strong. The wolf strained a bit at this one but soon broke it as well. Then the gods became more afraid of the wolf's power. Odin sent Skirnir, Freyr's messenger, down into the world of the dwarfs and had them fashion a magic restraint called Gleipnir ( See Fenrir's Binding )fs. It was made of six ingredients: the sound of a cat's footfall, the beard of a woman, the roots of a mountain, a bear's sinews, a fish's breath, and a bird's spittle. When it was done, Gleipnir was smooth and soft, like a silken ribbon. Skirnir brought it back to the home of the gods, and they took it to the island of Lyngvi by the lake Amsvartnir. They called the wolf, showed him the silky band, and challenged him to test his strength again.
Fenrir was suspicious because of the thinness of the band. The gods agreed to free him if he could not break out of the fetter himself, but Fenrir was still reluctant to have it put on him. He asked that someone put their hand into his mouth as a pledge that the gods were acting in good faith.
None of the gods was willing to take such a risk, knowing full well the deceit, but then Tyr stepped forward and put his right hand into the wolf's mouth, making the sacrifice that would keep the gods safe. Fenrir was bound with Gleipnir, and he tried with all his might but could not snap it. The gods laughed to see the wolf's distress--except for Tyr: Fenrir closed his mouth on Tyr's hand at the wrist.
Once the wolf was bound, the gods took a cord, called Gelgia, that hung from the fetter, and threaded it through a great stone slab called Gioll. They fastened the slab deep into the earth. Then they took a huge rock, called Thviti, using it for an anchoring-peg.
The wolf, in his anger, struggled violently and stretched its jaws frighteningly wide, trying to bite them all. The gods thrust a tall sword into Fenrir's mouth as a gum prop, with its hilt touching his lower gums and the point touching his upper gums. Fenrir continued to howl horribly, and saliva ran from his mouth. In this subdued condition, according to the myths, the terrible Fenriswolf would remain until Ragnarok, when the gods and the giants would fight to the death.