Mjölnir (ミョルニル, Myoruniru?) is the Divine Construct hammer of the Thunder God Thor, said to be the pinnacle of a mythos. It requires the belt Megingjörð and the protective gauntlet Járngreipr to wield it. Crafted by the dwarf brothers Brokkr and Sindri, Thor used it throughout his various exploits. It was once stolen by the king of the giants, Þrymr, but he never truly wielded it. After Thor died during Ragnarok, his two children, Móði and Magni, found the hammer upon the reborn earth, and they both can be considered to be its proper inheritors. Magni can wield the hammer without the gauntlet and belt.
It is utilized by Beatrice Flowerchild through the Berserker Class Card embodying Magni. It is a massive, ornate hammer that requires a massive arm of great strength in order to wield. It is a powerful melee weapon, and it can be thrown with great force. It attacks with the power of lightning, and shows multiple stages of power when the hammer's lightning is charged. As the power of the hammer grows, the triquetra symbol at the center also expands, the criss-crossing lines on the hammer extending out as jagged tendrils.
Mjölnir also gave Magni Thor's Divine Core when inheriting the hammer, allowing him the Authority of Thor and control over Mjölnir. If Thor's Divine Core is destroyed, the hammer will split and go out of control. It will return the surrounding area back to the Age of Gods and cause Ragnarök. The sky will turn red, the ground will be upturned, the Card holder's precious memories will be forsaken and turned into lightning to tear through the air, and all modern laws, be it the laws of physics or the modern Thaumaturgical Foundation, will no longer function; only magecraft from the Age of Gods, like Bazett's Runes, can be utilized. Even the previous wielder of Mjölnir will be struck by its lightning.
Mjölnir: All-Crushing Hammer of the Thunder God (悉く打ち砕く雷神の鎚, Kotogotoku Uchikudaku Raijin no Tsuchi?) is the first stage of the Noble Phantasm that releases a large lightning strike at the enemy. Requiring the power of lightning, the attack cannot be used at full power indoors, but still displays overwhelming power in such a state. Outside, lightning can be gathered to charge the weapon by raising it towards the sky, doubling its potential power. Its power is such that even High-Thaumaturgy casted by Illya while using the Caster Class Card can only hold out for a few seconds, eventually breaking through five gates of A-rank level thaumaturgy.
Mjölnir: Storm of the Thunder God Roaring with Thunder (万雷打ち轟く雷神の嵐, Banrai Uchi Todoroku Raijin no Arashi?) is the second stage of the Noble Phantasm, releasing twelve highly-dense lightning pillars from the sky that move along the ground to enclose the enemy in a circular pattern. The hammer takes in more lightning than the previous stage, causing the ornate markings to transform and extend from the hammer, tripling the amount of magical energy compared to the previous stage.
Mjölnir Ragnarök: Father's Lightning Burial Which Resounds Across the Twilight (黄昏に響け父の雷葬, Tasogare ni Hibike Chichi no Raisō?) is the final stage of Mjölnir. The hammer absorbs memories over time as the price for utilizing it, possibly because Magni is not the original wielder. It is a burial ritual intended by Thor as a his final message left to his children, those who will walk beyond the Twilight, the End. Although death is sad in and of itself, the existence of someone to mourn the death means the bonds will not fade, the feelings remaining even if their body is destroyed and the deceased erased from memory. The hammer, the proof Thor once lived, is returned to the heavens along with his memories, allowing those left behind to move forward.
The condition for activation is the destruction of Thor's Divine Core, causing Magni to become fundamentally different and lose authority to control the hammer. Mjölnir is destroyed, splitting open, and Magni loses all features of Thor. Upon activation, regardless of the climate, the sky will become crimson, and heavy rain and lighting will begin to fall. The possessor of Mjölnir will have the "memorial memories" of their lifetime become lighting and crash down on the battlefield. This begins to reenact the conditions of Ragnarök, causing the modern laws of the world to be negated as the area returns to that of the Age of Gods. Modern magecraft ceases to function, technology becomes inoperable, and the very laws of physics no longer apply, allowing phenomena such as debris floating in the absence of gravity. The lightning is indiscriminate, striking even Magni, and it causes those in the area to see memories of the user as they are forsaken. The memories are permanently lost after the Noble Phantasm is deactivated, but Beatrice is able to affirm herself and her feelings of love for Julian Ainsworth.