"Yashakiden: The Demon Princess" is a dark supernatural manga series set in a sprawling, dystopian city called Demon City Shinjuku, a place teeming with supernatural entities, occult phenomena, and human corruption. The narrative centers on the arrival of four enigmatic individuals—Aki, Kikiou, Ryuuki, and Shuuran—who are revealed to be ancient, powerful demons from China. These beings, known as the "Yasha," possess immense supernatural abilities and a thirst for domination. Their presence disrupts the fragile balance of power in Shinjuku, drawing the attention of both human and supernatural factions.

The story intertwines the Yasha's machinations with the lives of several key human characters. Chief among them is Mephisto, a mysterious and charismatic figure who operates as a doctor and occult researcher. Mephisto serves as a mediator between the human and supernatural worlds, often manipulating events to serve his own inscrutable goals. Another central figure is Setsura Aki, a young man with extraordinary abilities, including the power to manipulate razor-sharp wires. Setsura becomes a pivotal player in the conflict, as his skills and moral ambiguity place him at the crossroads of the Yasha's ambitions and the city's survival.

As the Yasha consolidate their power, they unleash a wave of terror across Shinjuku, targeting both humans and rival supernatural entities. Their ultimate goal is to establish a new order, one where they reign supreme. However, their plans are complicated by internal rivalries and betrayals, as well as the growing resistance from humans and other supernatural beings. The narrative delves into themes of power, immortality, and the blurred lines between good and evil, as characters navigate a world where survival often comes at a moral cost.

The series is marked by its intricate plotting, rich world-building, and a cast of morally complex characters. The Yasha are portrayed as both terrifying and tragic figures, their immortality and power isolating them from humanity while driving their relentless pursuit of dominance. The human characters, particularly Setsura and Mephisto, are equally layered, their motivations and allegiances shifting as the story progresses. The manga's dark, atmospheric tone is complemented by its exploration of occult lore and its depiction of a city where the boundaries between reality and nightmare are constantly eroded.