In Norse mythology, Fenrir or Fenrisulfr is a wolf, the son of Loki and the giantess Angrboða. Fenrir is bound by the gods, but is ultimately destined to grow too large for his bonds and swallow Odin whole during the course of Ragnarök. He will be slain by Odin's son, Viðarr, who will use a shoe made throughout time using the pieces of leather waste left over from repaired shoes to hold the lower jaw while he grasped the upper up, tearing the beast apart and avenging his father.
Fenrir has two sons, Hati ('hate') and Skoll. Skoll chases the horses Árvakr and Alsviðr, that drag the chariot which contains the sun. Hati chases Máni, the moon. 'Skoll', in certain circumstances, is used as a heiti to refer indirectly to the father (Fenrir) and not the son. This ambiguity works in the other direction also, for example in the Vafþrúðnismál, where a confusion exists in stanza 46 where Fenrir is given the sun-chasing attributes of his son Skoll. This can mostly be accounted for by the use of Hróðvitnir and Hróðvitnirson to refer to both Fenrir and his sons.
Learning from the prophecy of the sybil (cf. Völuspá) and from his contest with Vafþrúðnir (related in Vafþrúðnismál) that the children of Loki and Angrboða would bring trouble to the gods, Odin had the wolf brought to him along with his brother Jörmungandr and his sister Hel.
After casting Jörmungandr into the sea and Hel down into the land of the dead, Odin had the wolf raised among the Æsir. Only the god Týr was daring enough to feed the growing monster. The gods, urged by the wolf's increasing strength and by prophecies that he would be their destruction, attempted to bind the great beast. Twice he agreed to be chained and twice easily burst out of two successive fetters. The first, made of iron, was called Lœðingr. The second, also of iron, but of twice the strength, was called Drómi.
Odin then had the dwarfs forge the chain Gleipnir ("deceiver" or "entangler"). It appeared to be only a silken ribbon but was made of six wondrous ingredients: the sound of a cat's footfall, the beard of a woman, the roots of a mountain, bear's sinews (meaning nerves, sensibility), fish's breath and bird's spittle. The creation of Gleipnir is said to be the reason why none of the above exist. Skírnir, Freyr's messenger, brought it back to Ásgarðr.
Then, in the island called Lyngvi ("Heathery") in the lake called Ámsvartnir ("Red-black") (places unknown to us), the gods challenged Fenrisulfr to break this chain also. But the wolf noted the thinness and fineness of construction of Gleipnir and not unreasonably suspected a trick. He agreed to make the test only if one of the gods was willing to place his hand in the wolf's mouth during the binding as a pledge to free him if he failed to break the chain. No god was willing to do this, until Týr stood forth and placed his hand in the wolf's mouth. Fenrisulfr strained to burst the chain but the more he struggled the tighter he was held. When the gods would not free him, the wolf bit off Týr's hand at the wrist, the point afterwards called "the wolf joint". The wolf in his rage then tried to bite at the rest of the gods, but then they thrust a sword in his mouth to keep him from biting at his bonds.
Then, as told by Snorri in Brodeur's translation:
When the Æsir saw that the Wolf was fully bound, they took the chain that was fast to the fetter, and which is called Gelgja 'Thin', and passed it through a great rock—it is called Gjöll 'Scream'—and fixed the rock deep down into the earth. Then they took a great stone and drove it yet deeper into the earth—it was called Thviti—and used the stone for a fastening-pin. The Wolf gaped terribly, and thrashed about and strove to bite them; they thrust into his mouth a certain sword: the guards caught in his lower jaw, and the point in the upper; that is his gag. He howls hideously, and slaver runs out of his mouth: that is the river called Ván 'Hope'; there he lies till the Twilight of the Gods.
It is prophesied that at Ragnarök the wolf will at last break free and join forces with the enemies of the gods and will then swallow Odin himself whole. After that Viðarr, Odin's son, will slay the wolf to avenge his father.