Fenrir (Old Norse, Fenrisúlfr) is the first child of the god Loki and the giantess Angrboda in Norse mythology before Hel and Jormungandr. Other siblings are Odin's eight-legged horse Sleipnir, Narfi and Vali.
Fenrisúlfr (Old Norse: fen for "swamp") literally means "swamp wolf". Other names for him are: Fenrir, the swamp-dwelling one, Vángandr, monster by the river Ván, or Hróðvitnir, the famous wolf (vitnir is a kenning for wolf. This refers to the idea that witches ride wolves and goes back to the Old Norse word vitt "sorcery, witchcraft").
The gods recognized the danger posed by Fenrir and brought it to Asgard to keep an eye on him. At first he was a harmless cub, but as he grew larger and stronger each day, the gods felt threatened: they feared he would devour them all.
So they decided to chain him up forever. First they had two heavy chains made (Läding, the one that binds with cunning, and Droma, the one that restrains), but the wolf broke them effortlessly. Now he was to test his strength on the magic shackle Gleipnir, which looked as harmless as a simple thread.
However, the thread was made by the dwarves from the things that do not exist. For example, from the sinews of bears, the voice of fishes, the beards of women, the saliva of birds, the sound of a cat's footsteps and the roots of mountains.
Fenrir became suspicious. He demanded as a pledge in case of a possible deceit that one of the gods put his right hand in his mouth. Nobody wanted to give himself for it, except Tyr, the god of the war and justice.
So they tied Fenrir, but the harder he tore at the shackle, the tighter it tightened. Fenrir remained captive, but bit off Tyr's right hand in revenge.
Thus the world of the gods was safe from the wolf, but Tyr's right hand could not be saved, which is why he was known only as the One-Handed Aesir (Old Norse einhendr asa).
The giant wolf will free himself only in the time of the end of the world, called Ragnarök ("fate of the gods"). He will then devour Odin and as a result will ultimately be killed by Odin's son Vidar in a duel.