A forgotten temporal guardian, known only as Zero, awakens in the Demon Slayer world after centuries of dormancy, his immense power suppressed by dimensional barriers. Dressed in an alien attire and accompanied by his tiny robot companion Xiao Yue, he moves through the rural landscapes of Nihon with eerie calm, analyzing every threat with surgical precision. When he encounters a demon, he doesn’t rush—he observes, tests, and waits, using advanced tech and centuries of cosmic combat experience to dismantle foes with terrifying efficiency.

His unorthodox methods draw the attention of the Demon Slayer Corps, especially the Butterfly Sisters, who watch in awe as he defeats a Lower Tier demon without a Nichirin Blade. To them, he’s a mysterious, overcautious pillar—strong beyond comprehension, yet unnervingly hesitant to act. As Zero pieces together fragments of his lost past, he begins to suspect the demons are not the only threat lurking in this world…

and that his own awakening may have triggered something far older and deadlier.