"Dies irae" is a supernatural action anime based on the visual novel of the same name. The story is set in modern-day Japan and revolves around a group of individuals entangled in a conflict involving ancient relics, occult rituals, and a secretive organization known as the Longinus Dreizehn Orden. The narrative begins with Ren Fujii, a high school student in Suwahara City, who becomes embroiled in a series of violent events after witnessing a murder committed by his friend Shirou Yusa. Shirou, driven by a desire to resurrect a powerful being, sacrifices himself in a ritual, setting off a chain of events that draws Ren into a world of supernatural warfare.

Ren discovers that he possesses a unique ability tied to the "Obsidian Round Table," a group of individuals who wield supernatural powers derived from the Holy Relics, artifacts connected to the Spear of Longinus. These relics grant their users immense power but come at the cost of their humanity. Ren is pursued by members of the Longinus Dreizehn Orden, a cult-like organization seeking to use the relics to summon the god-like being known as Mercurius and bring about the end of the world. The Orden is led by Reinhard Heydrich, a charismatic and ruthless figure who commands a group of powerful individuals, each with their own motivations and connections to the relics.

As the story progresses, Ren allies with Kasumi Ayase, a childhood friend who also becomes involved in the conflict, and Marie, a mysterious girl with ties to the relics and the Orden. Together, they confront the Orden's members, including figures like Wilhelm Ehrenburg, a vampire-like warrior, and Rusalka Schwägerin, a tragic figure bound to Reinhard. The narrative delves into themes of fate, power, and the consequences of wielding god-like abilities, as Ren and his allies struggle to prevent the Orden from achieving their apocalyptic goals. The story builds toward a climactic confrontation between Ren and Reinhard, with the fate of the world hanging in the balance.

The anime explores the complex relationships between its characters, their pasts, and their motivations, often blurring the lines between hero and villain. The plot is driven by a mix of intense action sequences, philosophical undertones, and intricate lore surrounding the relics and their origins. While the series condenses the source material, it retains the core elements of the visual novel, including its focus on the characters' struggles and the high-stakes conflict at the heart of the story.