In Norse mythology, Fenrir is a wolf. Fenrir was the eldest of three children between Loki and the giantess Angrboda. The Gods kept Fenrir in Asgard so that they might keep an eye on him.
When he was still a pup they had nothing to fear, but when the Gods saw one day how he had grown, they decided to render him harmless. However, none of the Gods had enough courage to face the gigantic wolf. Instead, they tried to trick him. They said the wolf was weak and could never break free when he was chained. Fenrir accepted the challenge and let the Gods chain him with iron. Unfortunately, he was so immensely strong that he managed to break the strongest fetters as if they were cobwebs.
The Gods learned of a prophecy which stated that the wolf and his family would one day be responsible for the destruction of the world. They caught the wolf and locked him in a cage. Fenrir had grown so large that when he opened his mouth, his lower jaw would be against the Earth and his upper jaw would scrape the universe. Flames burned from his eyes and nostrils. Only God Tyr was brave enough to feed him.
The Gods saw only one alternative: a magic chain (killing was forbidden in Asgard). They ordered the dwarves to make something so strong that it could hold the wolf. The result was a soft, thin ribbon: Gleipnir. It was incredibly strong, despite what its size and appearance might suggest. The ribbon was fashioned of six elements: the footstep of a cat; the roots of a mountain; a woman's beard; the breath of fishes; the sinews of a bear; and a bird's spittle. That is why these things are very rare in the world today.
The Gods tried to trick the wolf again, only this time Fenrir was less eager to show his strength. He saw how thin the chain was and said that was no pride in breaking such a weak chain. Eventually, though, he agreed, thinking that otherwise his strength and courage would be doubted.
Suspecting treachery however, he in turn asked the Gods for a token of good will: one of them had to put a hand between his jaws. The Gods were not overly eager to do this, knowing what they could expect. Finally, only Tyr agreed, and the Gods chained the wolf with Gleipnir. No matter how hard Fenrir struggled, he could not break free from this thin ribbon. In revenge, he bit off Tyr's hand.
Being very pleased with themselves, the Gods carried Fenrir off and chained him to a rock, Gioll, a mile down into the Earth. When Fenrir howls in his anger, the ground and mountains tremble violently and deep fractures form and rocks were thrown around. They put a sword between his jaws to prevent him from biting. On the day of Ragnarok, Fenrir will break his chains and join the giants in their battle against the Gods. He will seek out Odin and devour him. Odin's son Vidar will avenge his father by killing the wolf. Vidar will grasp his jaws with one hand and tear his mouth apart, killing the beast at last.