"Dies irae ~Amantes amentes~" is a dark fantasy manga adaptation of the visual novel of the same name, blending historical, supernatural, and philosophical themes. The story is set in modern-day Japan but intertwines with events from World War II, particularly involving the occult experiments conducted by the Thule Society, a real-world Nazi occult group. The narrative centers on a group of individuals who become entangled in a supernatural conflict over the creation of a god-like entity known as the "Einherjar."

The protagonist, Ren Fujii, is a high school student with a troubled past, marked by violence and a strained relationship with his family. His life takes a drastic turn when he encounters a series of supernatural events, including the reappearance of his childhood friend, Shirou Yusa, who had been presumed dead. Shirou is revealed to be one of the key figures in the occult conflict, having been resurrected as a vessel for the Thule Society's experiments. Alongside Ren is Marie, a mysterious girl with ties to the supernatural, who becomes a central figure in the unfolding events.

The plot revolves around the "Longinus Dreizehn Orden," a secretive organization seeking to complete the "Reinhard Heydrich," a ritual to create the Einherjar. This entity is intended to transcend human limitations and achieve godhood. The organization is led by Reinhard Heydrich, a charismatic and ruthless figure inspired by the historical Nazi officer of the same name. Heydrich is supported by his loyal followers, known as the "LDO," each of whom wields unique supernatural abilities tied to their personal obsessions and desires.

As the story progresses, Ren and his allies are drawn into a series of battles against the LDO members, each confrontation revealing deeper layers of the characters' motivations and the philosophical underpinnings of their actions. Themes of power, morality, and the nature of humanity are explored through the characters' struggles, particularly as they grapple with the consequences of their choices and the allure of transcendence.

The narrative is structured around the concept of "Atziluth," a state of divine power that the characters strive to achieve or resist. This pursuit leads to intense conflicts, both physical and ideological, as the characters confront their own limitations and the ethical implications of their actions. The story culminates in a climactic showdown between Ren and Reinhard, with the fate of humanity hanging in the balance.

Throughout the series, the manga delves into the backstories of its characters, providing context for their actions and motivations. Ren's journey is one of self-discovery and redemption, as he seeks to reconcile his violent past with his desire to protect those he cares about. Marie's role as a catalyst for the events is gradually revealed, tying her fate to the larger conflict. The LDO members, each with their own tragic histories, serve as both antagonists and mirrors to the protagonists, highlighting the complexities of human nature.

"Dies irae ~Amantes amentes~" is a dense and intricate narrative, combining action, drama, and philosophical inquiry. Its exploration of historical and supernatural elements creates a unique backdrop for its character-driven story, making it a compelling read for fans of dark fantasy and psychological drama.