The half-god wolf Fenrir, son of Loki, was foretold to kill Odin at a great battle at the end of time. As the wolf grew larger and larger the gods felt threatened, and none of them dared to challenge the wolf. They decided to shackle him instead, but of course Fenrir would never agree to wearing a chain voluntarily, so the gods hatched a plan to trick the wolf.
The gods told Fenrir that he would become famous everywhere for his strength, if he could manage to break the strongest chain in the world. They tied him up with a great chain, and Fenrir snapped it easily. Then they made a chain that was twice as strong, and Fenrir ripped it apart like it was made of straw. One last time the gods issued the same challenge, but this time they presented Fenrir with a silken ribbon.
Fenrir was a smart wolf, and he was afraid the gods were trying to trick him somehow. So he told them that the only way he would allow anyone to fasten the ribbon around his neck was if the gods would show a token of good faith. One of the gods would have to hold their hand inside Fenrir's mouth, so he could feel safe that there was no foul play involved.
Since the ribbon was actually a magical chain made by dwarven artisans, there was indeed foul play involved, so none of the Gods were willing to hold their hand inside Fenrir's mouth. After much discussion, Tyr agreed with the wolf's demands. He put his hand inside of Fenrir's mouth, while the other gods fastened the ribbon around the wolf. At first, Fenrir thought the ribbon was weak, and that he'd be able to break it easily. But as he kept trying to get himself free, the ribbon grew stronger and stronger, tighter and tighter, and soon it was too late. Fenrir realised he had been tricked, and he angrily snapped his mouth shut and bit Tyr's hand right off! But the gods had the wolf trapped, and they bound him to a great mountain.
Tyr was hailed for his bravery and sacrifice, and Fenrir was chained there till Ragnarok, the end of the world.
