Halcyon is a Renaissance Hardcore composition by xi (Yusuke Ishiwata) that originated in the BMS (Be-Music Source) doujin scene. It was the top-rated entry in THE BMS OF FIGHTERS 2010 (BOF2010), claiming first place in the individual score division and second in the individual median division, a historic achievement that cemented its status as one of the most celebrated BMS tracks of its era. The song draws its name and thematic weight from Greek mythology. "Halcyon" refers to the kingfisher bird (翡翠鳥, kawasemi), which traces back to the myth of Alcyone (アルキュオネー), daughter of the wind god Aeolus (アイオロス). In the legend, Alcyone and her husband Ceyx were transformed into kingfishers, giving rise to the term "halcyon days" for the calm winter period when the birds were said to nest on the sea. The in-game video, created by yama_ko, visually echoes this mythological narrative. Halcyon landed in maimai でらっくす on July 19, 2019 under the GAME & VARIETY (ゲーム＆バラエティ) category. Two years later, Standard (ST) charts arrived with UNiVERSE PLUS, initially unlockable through Friend Matching Season 4 (300 SP) before becoming generally available when FESTiVAL launched. The song's signature musical feature is its dramatic BPM architecture: while listed at 191 BPM, the intro hovers around 95.5 BPM and then gradually decelerates to a crawl of 40 BPM before the chorus erupts at full speed. Slide notes are placed through this slowdown section, testing players' sense of timing at unusually low tempos. The DX Master chart (designed by ぴちネコ / pichineko) sits at level 13 with a constant of 13.5, while the Standard Master chart (designed by ロシアンブラック / Russian Black) reaches level 14 with a constant of 14.3, placing it among the more demanding Standard charts of its version. The DX and Standard Expert charts, designed by うさぎランドリー (Usagi Laundry) and 隅田川星人 (Sumidagawa Seijin), respectively, offer their own challenges at levels 11 and 12. Halcyon also appears in sibling SEGA rhythm games CHUNITHM and オンゲキ, and a narrative sequel titled "Aiolos" (named after Alcyone's father) later appeared in maimai ORANGE PLUS, expanding the mythological thread.