Mjolnir

Mjölnir is the Hammer of the the Lightning God of Norse, Thor. A replica version of it was temporarily lend to Issei Hyoudou.

Appearance
The Mjölnir is a giant hammer with a black outer frame with a blue structure throughout its entire design from the head to the handle. There is a Norse symbol on its middle piece. In the anime, when having its power boosted by Issei's Boosted Gear, the Mjölnir gains glowing design expanding throughout the hammer.

Light Novel version
The original Mjölnir doesn't appear in the Light Novels but a replica of the Mjölnir was handed to Issei to assist him in combating Loki and Fenrir. Despite being a replica, it retains the ability as the actual one causing Issei to originally faced difficulties in using the Mjölnir replica due to its sheer weight. This was overcome however when Issei used Bilingual on Akeno and accidentally gained the blessing of Chichigami allowing him to use the Mjölnir replica and defeat Loki after striking him with the lightning produced by the replica boosted by the Boosted Gear.

Anime version
The real Mjölnir is summoned by Odin to the battlefield where the young Devils, Irina and Rossweisse are battling Loki and his monsters. Like in the novels, the Mjölnir is handed to Issei who transfers the Boosted Gear's power into the hammer and striking Loki with the hammer defeating him.

Abilities
Mjölnir is a weapon that only a God can wield. The Hammer is capable of releasing powerful lightning and thunder attacks capable of defeating a God. The replica of the Mjölnir lend to Issei also displayed the same abilities as original one The replica is capable of increasing or shrinking in size when manipulated with Demonic powers.

While not shown to have a shrinking ability, in the anime, the Mjölnir, after being boosted by the Boosted Gear, cam increase in size when attacking.

Trivia

 * In the anime, Issei never face any difficulties in using the Mjölnir.
 * In the anime, the Mjölnir never released any thunder or lightning.