Senkō no Buryūnaku (閃鋼のブリューナク, "Flashing Steel Brionac") is a high-energy original composition by sasakure.UK, best known to rhythm game fans for his numerous contributions across BMS, CHUNITHM, maimai, and other arcade music games. The track originates from SEGA's CHUNITHM and was brought into maimai during the PiNK era as part of the first wave of Challenge Tracks, a limited-time unlock feature that had players clearing specific conditions to access the song before its general release in January 2016. The vocal performance is provided by ピリオ (Period), a deliberately mysterious singer whose identity is kept veiled; within CHUNITHM, ピリオ is also represented as a playable character, tying the song closely to that game's lore and worldbuilding. The title alludes to Brionac (sometimes spelled Brionaic), the legendary spear of the Celtic god Lugh, and the lyrics and instrumental arrangement evoke a sense of mythic battle and techno-medieval grandeur. In maimai, the song's charts underwent several level revisions across version updates, reflecting its evolving reputation as a demanding track particularly at the MASTER difficulty, which was originally a level 12 before climbing to 13+ in later versions. The jacket artwork and in-game video were produced by HDLV, whose distinctive visual style complements the song's sword-and-sorcery motif.