After being hospitalized for two months due to a violent incident with his best friend, Ren Fujii just wants to return to a peaceful life in Suwahara City. But his attempt at normalcy quickly unravels. As winter sets in, Ren begins having vivid nightmares filled with brutal duels and mysterious figures cloaked in blood and fire. At the same time, a series of bizarre events—including inexplicable murders and strange individuals appearing around town—suggests that something dark is brewing beneath the surface of his everyday life. The common route follows Ren’s slow descent into a world he doesn’t understand, where unseen forces manipulate events from the shadows and his own reality begins to fracture. As he tries to protect those close to him, Ren is forced to confront a truth far greater—and more terrifying—than anything he imagined.

Ren Fujii’s attempt to reclaim a normal life is complicated not only by disturbing dreams and supernatural phenomena, but also by the presence of Kasumi Ayase—his childhood friend and one of the few people still trying to pull him back into everyday life. In her route, the focus shifts to their strained but deep connection as Kasumi becomes more involved in the dark events surrounding Ren. As tensions in the city escalate and otherworldly threats close in, Kasumi is drawn into the conflict in unexpected ways.

As strange events in Suwahara spiral further out of control, Ren finds himself crossing paths with Kei Sakurai—an elite warrior aligned with the mysterious Longinus Dreizehn Orden, the group behind the supernatural chaos engulfing the city. Unlike other members of the Orden, Kei shows a surprising level of restraint and independence, revealing a conflicted loyalty and a strict personal code rooted in her past. In this route, Ren is thrust deeper into the heart of the unfolding war. Rather than simply opposing the enemies around him, he begins to understand their ideologies and inner struggles, especially Kei’s. Their encounters become a test of conviction and humanity as both characters are forced to confront not just each other, but their own roles in the larger scheme threatening the world.

As the surreal nightmares and violent incidents consuming Ren’s world intensify, he encounters Marie—a mysterious, ethereal girl who seems intimately connected to his visions of war and death. Unlike the grounded tension of the Kasumi route or the ideological warfare of Kei’s, Marie’s path delves deep into the metaphysical and emotional heart of Dies irae. Her presence feels both familiar and distant, as if she’s a part of Ren’s soul that has long been forgotten.

Rea Himuro’s route serves as the grand finale of Dies irae, where all the narrative threads from the previous routes—Kasumi, Kei, and Marie—converge into a complete and climactic whole. As the supernatural war reaches its peak, Ren is thrust into the heart of the conflict, confronting the ultimate truths behind the Longinus Dreizehn Orden, their apocalyptic ideology, and the terrifying will that drives them forward. Rea, a cool and commanding presence within the enemy’s ranks, emerges as a pivotal figure—one whose motivations and connections challenge Ren’s understanding of friend and foe alike. This route delves deep into the philosophical core of the story: the nature of free will, eternity, sacrifice, and what it means to live with purpose in a world on the brink of collapse.