Fenrir (or Fenris) is a gigantic and terrible monster in the shape of a wolf. He is the eldest child of Loki and the giantess Angrboda. 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. Only the god of war, Tyr, dared to feed and take care of the wolf. 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. Unfortunately, he was so immensely strong that he managed to break the strongest fetters as if they were cobwebs. After that, the gods saw only one alternative left: a magic chain. 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 strange 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. 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 (called Gioll) a mile down into the earth. 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. Vidar, Odin's son, will avenge his father by killing the wolf.

*The above is from the Encyclopedia Mythica: http://www.pantheon.org/articles/f/fenrir.html