Re:Unknown X is a piano-driven Touhou arrangement by prolific pianist and NicoNico Douga musician marasy (まらしぃ). The track reworks "Unknown X ~ Unfound Adventure", the final boss theme from the fighting game Touhou Hisouten ~ Scarlet Weather Rhapsody (東方非想天則), composed by ZUN of Team Shanghai Alice. marasy's arrangement recasts the battle theme into a virtuosic piano piece that retains the original's intensity while showcasing his signature rapid-keyboard style at a steady 190 BPM. The song was originally commissioned as an original composition for Touhou Danmaku Kagura (東方ダンマクカグラ), a mobile rhythm game that ran from 2021 to 2022. Re:Unknown X holds the peculiar distinction of being the very last song added to that game before its service ended, it dropped on October 13, 2022, just two weeks before the shutdown on October 28. In Danmaku Kagura, its Lunatic difficulty was rated with the fearsome "X" mark, cementing it as that game's most demanding chart. The "Re:" prefix in the title was chosen by marasy as a deliberate reference to email reply headers, a gesture of respect toward the game's development collective, which was itself named "Unknown X." After Danmaku Kagura's closure, the song was carried into the rebuilt successor Phantasia Lost (ファンタジア・ロスト), where it appeared in teaser trailers and once again occupied the highest difficulty tier. In maimai, Re:Unknown X arrived as part of the BUDDiES PLUS update on June 7, 2024, through a limited-time Touhou Danmaku Kagura Phantasia Lost Area collaboration. Its MASTER chart, designed by アミノハバキリ (Aminohabakiri), became the very first chart rated 14+ in the 東方Project category, a milestone for the genre within the game. Players familiar with the chart describe it as heavily stream-oriented, with dense note slides and a notorious 24th-note trill passage near the end (equivalent to 16th notes at an effective BPM of 285) that demands precise hand coordination.