Gleipnir

Gleipnir are mystical chains forged by Dwarves used to bind Fenrir.

Appearance
Gleipnir are very big chains. In the anime they also appear to be glowing.

Summary
The mystic chains of Gleipnir were given Issei Hyoudou and Vali's comrades in the final fight against Loki and Fenrir under the advise of Midgardsormr. Because of how heavy they were, the magical chains to Kuroka to keep them in her personal territory.

As the fight with Loki had commenced, Kuroka summoned the chains to ensnare Fenrir, while Loki had prepared for them, he did not anticipate that they would move on their own as Fenrir was eventually captured.

Abilities
Gleipnir were forged to be unbreakable to whoever they were binded to, especially the God-Devouring Wolf; Fenrir, they were also incredibly heavy and nearly impossible to carry. These chains were also enhanced by Dark Elves to wrap themselves its target as if it had its own consciousness.

Trivia

 * In Norse myth, Gleipnir were used to bind Fenrir by the Norse Gods who feared him and would hold him until Ragnarök, when he would break free and devour Odin.