"Requiem from the Darkness" is a dark, supernatural anime set in feudal Japan, blending historical elements with horror and mystery. The narrative centers on Momosuke, a young aspiring writer who seeks to compile a collection of supernatural tales titled "Hundred Stories." His journey leads him to encounter a mysterious trio known as the Ongyou, composed of Mataichi, a stoic and enigmatic leader; Ogin, a seductive and cunning woman; and Nagamimi, a grotesque and unsettling figure. The Ongyou are supernatural investigators who punish those who commit heinous crimes, often using their own dark abilities to deliver justice.

As Momosuke delves deeper into his research, he becomes entangled in the Ongyou's missions, witnessing their brutal methods of retribution. Each episode explores a standalone story, often involving vengeful spirits, cursed artifacts, or morally corrupt individuals. These tales are steeped in Japanese folklore and mythology, with themes of karma, retribution, and the consequences of human cruelty. Momosuke's initial fascination with the supernatural gradually gives way to horror as he realizes the true nature of the Ongyou and the darkness they embody.

The series maintains a grim and atmospheric tone, with its episodic structure allowing for a variety of haunting and tragic narratives. Momosuke serves as both a protagonist and an observer, providing a human perspective on the Ongyou's actions and the moral ambiguities of their justice. The interplay between Momosuke and the Ongyou evolves over the course of the series, revealing deeper layers of their motivations and the tragic histories that bind them. The anime concludes with a sense of inevitability, as Momosuke's fate becomes intertwined with the supernatural forces he sought to document.