The Mjölnir replica was also able to increase and decrease in size after the wielder's aura is imbued into it as shown when Issei channeled his demonic power in it, while it is unknown if the original can do the same, it can be assumed. The replica of Mjölnir, which was lent to Issei has displayed the same lightning-based abilities as the original, as well as having power close to the original. Mjölnir is one of the weapons that can only be wielded by gods, which is capable of generating and controlling thunder and lightning capable of defeating other gods. Mjolnir after being charged up with the Boosted Gear

Like in the novels, the Mjölnir is handed to Issei who transfers the Boosted Gear's power into the hammer, striking Loki with the hammer and defeating him. The real Mjölnir is summoned by Odin to the battlefield where the young Devils, Irina and Rossweisse are battling Loki and his monsters. In Volume 13, it was explained by Surtr Second that Mjölnir was forged by the Dwarves from the Ívaldi Tribe for Thor. The hammer itself is light as a feather and bears no weight, except for those who have any evil feelings, as the hammer can only be wielded by those with a strong and pure heart. However, this was overcome when Issei used Pailingual on Akeno to find out about her past and contacted the Chichigami by accident, who later granted him a blessing that allowed him to use the Mjölnir replica and defeat Loki, after striking him with the lightning produced by the replica boosted by the Boosted Gear. Despite this aid, Issei (even in Scale Mail armor) faced difficulties in using the Mjölnir replica due to its sheer weight. Despite being a replica, it retains the same abilities as the original, however, as it is still classified as a divine weapon it couldn't be used by Issei without the initial assistance of Baraqiel and other people. The original Mjölnir doesn't appear in the Light Novels, instead, a replica was handed over to Issei to assist him in combating Loki and Fenrir.

In the anime, when having its power boosted by Issei's Boosted Gear, the Mjölnir gains a glowing design expanding throughout the hammer.

There is a Norse symbol on its middle piece. Mjölnir is a giant hammer that has a black outer frame with a blue structure throughout its entire design from the head to the handle.

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