Gleipnir

In Norse mythology, Gleipnir (Old Norse "open one") is the binding that holds the mighty wolf Fenrisulfr (as attested in chapter 34 of the Prose Edda book Gylfaginning). The Gods had attempted to bind Fenrir twice before with huge chains of metal, but Fenrir was able to break free both times. Therefore, they commissioned the dwarves to forge a chain that was impossible to break. To create a chain to achieve the impossible, the dwarves fashioned the chain out of six impossible things: