Neither iron nor chains tamed the mightiest monster of the North, but a ribbon thin and soft as a silken thread: Gleipnir, the magic fetter that holds the Fenris-wolf.
Fenrir had snapped two heavy iron chains with ease. So the gods had the dwarves in Svartalfheim forge a new band – from six things that do not exist: the sound of a cat’s footfall, the beard of a woman, the roots of a mountain, the sinews of a bear, the breath of a fish and the spittle of a bird. That, the Edda says with a wink, is exactly why these things no longer exist today: they are all inside Gleipnir.
Fenrir smelled the trick and would only be bound if a god laid his hand in the wolf’s mouth as a pledge. Only the war-god Tyr dared. When the wolf found he could not break free, he bit down – and Tyr lost his right hand. So Gleipnir holds the wolf until Ragnarök, when he tears loose and devours the world.
Gleipnir is the third and last fetter. Fenrir, the wolf-son of Loki, grew up among the gods – but a prophecy warned that he would one day bring their doom. So they tried to bind him. The first chain Leyding he burst at the first twitch, the second, stronger Dromi likewise. Only the slender, unremarkable band from the dwarven forge held.
On the island Lyngvi in the lake Amsvartnir the wolf was bound; the end of Gleipnir the gods drew through a stone slab and fixed deep in the earth. A sword they set between his jaws so he could not bite. So Fenrir lies bound – until he tears loose at Ragnarök and devours Odin. The strongest fetter in the world is, in the end, only a reprieve.
The Fenris-wolf · Tyr · The Dwarves · Ragnarök.
Snorri Sturluson, Gylfaginning (34). Rudolf Simek, Dictionary of Northern Mythology.