In 2189, humanity survives between Heaven, the Abyss, and a fractured Mortal Realm. Kairo, a legendary assassin of the Veiled Order, leads Operation Hollow Saint beneath ruined Elaris, unaware that his squad has been ordered to sacrifice him and awaken Azrathiel, an ancient angel-demon catastrophe. Betrayed by the only family he trusts, Kairo must survive long enough to uncover why Heaven once feared humanity.