Fenrir is a monstrous wolf of Norse myth, one of the children of Loki by the giantess Angrboda, sibling to the world-serpent Jormungandr and to Hel. The Prose Edda of Snorri Sturluson tells the binding in detail: the gods raise the wolf themselves, find him growing beyond safety, and fail twice to hold him with ordinary fetters before the dwarves forge Gleipnir, a silken-seeming ribbon made of impossible things, the sound of a cat's footfall, the beard of a woman, the roots of a mountain, and three more.
Suspecting a trick, Fenrir agrees to be bound only if a god will place a hand in his mouth as a pledge; Tyr does so, and loses the hand when the wolf cannot break free. Fenrir stays bound until Ragnarok, when he breaks loose and swallows Odin, and is killed in turn by Odin's son Vidar. The tale is one of the clearest Norse statements of a recurring theme: the powers of destruction can be delayed by cunning and sacrifice, but not finally undone.