"Mission of Darkness" is a dark fantasy anime series set in a world where humanity is on the brink of extinction due to the rise of malevolent entities known as Shadows. These creatures, born from human despair and negative emotions, consume entire civilizations, leaving only ruins in their wake. The story centers on a group of individuals known as the "Harbingers," who possess unique abilities to combat the Shadows. Each Harbinger is bound by a mysterious artifact called the "Eclipse Shard," which grants them immense power but at the cost of their own life force.

The protagonist, Kael, is a former soldier who becomes a Harbinger after losing his family to the Shadows. Driven by vengeance and guilt, Kael joins forces with other Harbingers, including Lyra, a skilled tactician with a tragic past, and Riven, a rogue mercenary seeking redemption. Together, they embark on a perilous journey to uncover the origins of the Shadows and locate the mythical "Obsidian Core," said to be the source of their power. Along the way, they face not only the relentless Shadows but also internal conflicts, as the burden of their powers and the sacrifices required to wield them take a toll on their humanity.

The narrative is structured around episodic missions, each revealing more about the world’s lore and the characters’ backstories. Key plot points include the discovery of a hidden organization manipulating the Shadows for their own gain, the revelation of Kael’s connection to the Obsidian Core, and the moral dilemmas faced by the Harbingers as they grapple with the cost of their mission. The series explores themes of sacrifice, redemption, and the blurred line between light and darkness, culminating in a climactic battle that forces the characters to confront their deepest fears and desires.