In Norse mythology, Gleipnir (Old Norse for "open one") is the enchanted rope that is tied around Fenrir (as attested in Chapter 34 of the Prose Edda book Gylfaginning).
The gods attempted to bind the wolf twice before with large metal chains but both attemptes ended with the chains broken beyond repair. They commissioned the Dwarves of Svartálfaheim to forge an unbreakable chain. To create this the the dwarves used six impossible ingredients
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
The spittle of a bird
Therefore, even though Gleipnir is as thin as a silken ribbon, it is tronger than any chain. Fenrir sensed the gods' deceit and refused to be bound with it unless one of them put his hand in the wolf's mouth. Tyr known for his great wisdom and courage, agreed, and the other gods bound the wolf. After Fenrir had been bound by the gods, he struggled to try to break the rope. When Fenrir found he could not break the ribbon he bit off Tyr's hand. As a result of this deed, Tyr is called the "Leavings of the Wolf". Gleipnir is said to hold until Ragnarök, when it will break and Fenrir will be free to devour Odin and be killed himself by .