Gleipnir

Gleipnir is the enchanted rope that is tied around Fenrir.
Gleipnir is a magical relic item obtained from slaying the Fenrir.
Myths & Legends
The gods attempted to bind the wolf twice before with large metal chains but both attempts 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

Even though Gleipnir is as thin as a silken ribbon, it is stronger than any chain.
Gleipnir
Magic Binding: This rope can bind any being regardless of its size and type. It can bind titans and gods. 
Infinite Length: If being used to bind a creature, the rope can extend to size of creature in order to bind it. (Other uses of rope it is 50 ft.)

• Rope, Hempen 50' 
• Legendary Artifact
• Attunement by a creature with proficiency in Acrobatics or Athletics.
• 10 lbs

Mechanics & Inner Workings

Attunement
Some magic items require a creature to form a bond with them before their magical properties can be used. This bond is called attunement, and certain items have a prerequisite for it. If the prerequisite is a class, a creature must be a member of that class to attune to the item. (If the class is a spellcasting class, a monster qualifies if it has spell slots and uses that class's spell list.) If the prerequisite is to be a spellcaster, a creature qualifies if it can cast at least one spell using its traits or features, not using a magic item or the like. Without becoming attuned to an item that requires attunement, a creature gains only its nonmagical benefits, unless its description states otherwise. For example, a magic shield that requires attunement provides the benefits of a normal shield to a creature not attuned to it, but none of its magical properties.
Attuning to an item requires a creature to spend a short rest focused on only that item while being in physical contact with it (this can't be the same short rest used to learn the item's properties). This focus can take the form of weapon practice (for a weapon), meditation (for a wondrous item), or some other appropriate activity. If the short rest is interrupted, the attunement attempt fails. Otherwise, at the end of the short rest, the creature gains an intuitive understanding of how to activate any magical properties of the item, including any necessary command words.
An item can be attuned to only one creature at a time, and a creature can be attuned to no more than three magic items at a time. Any attempt to attune to a fourth item fails; the creature must end its attunement to an item first. Additionally, a creature can't attune to more than one copy of an item. For example, a creature can't attune to more than one ring of protection at a time.
A creature's attunement to an item ends if the creature no longer satisfies the prerequisites for attunement, if the item has been more than 100 feet away for at least 24 hours, if the creature dies, or if another creature attunes to the item. A creature can also voluntarily end attunement by spending another short rest focused on the item, unless the item is cursed.

Item type

Unique Artifact

Rarity

Legendary

Weight

10 lbs

Base Price

50,000 gp