Mjölnir is a thunderous piano-driven hardcore track by xi (Yusuke Ishiwata), added to maimai でらっくす UNiVERSE PLUS on May 13, 2022 as part of the Diverse System collaboration area. The song originally appears on the 2021 compilation album AD:PIANO ff/pp, released by the doujin music circle Diverse System. This album is a spin-off of the long-running AD:PIANO series (since 2012), which exclusively features piano-centric compositions, AD standing for "Advantage." What sets ff/pp apart is its dual-disc concept: eleven artists each contributed one piece for Disc 1 (ff, fortissimo, "very loud") and one for Disc 2 (pp, pianissimo, "very soft"), with Mjölnir anchoring the aggressive ff side. The concept was originally proposed by fellow composer Feryquitous. xi himself has described Mjölnir as a spiritual successor to his earlier track Longinus, which appeared in AD:PIANO V Noir and the mobile rhythm game Deemo. At 222 BPM with a MASTER chart rated 14.4, Mjölnir stands as a demanding boss chart within its GAME & VARIETY category, challenging players with dense note patterns and a notoriously tricky mid-section featuring short HOLD notes with no end judgment and poor visual readability.