The Underground is a mysterious and powerful faction that has shaped the history of DisCity since time unknown and is embroiled in a long-running conflict with Paradeisos for the fate of the city. It is a cult-like organization that appears to worship the Great Existence and seeks to use the power of Mania for their own ends. Although almost nothing is known about their origin or their goals, there is evidence that the Underground (or their predecessors) have existed since before Nightfall (perhaps even being responsible for the Apocalypse). The In Host's Name crimebrand may depict the origin of this faction. The Underground's influence is chiefly symbolized by individuals in black raincoats and umbrellas, as well as the insignia of a fiery, vertical-aligned eye.
The organizational structure, aims, and methods of the Underground are all unclear. Bai Yi refers to the existence of an "Underground city" that she and Che escaped from at some point in the past, which is likely the Underground's base of operations and may suggest that the faction is far larger than the small number of operatives who have been seen so far on the surface. Most of the Underground's work on the surface is performed by field agents like Parma and Segal, who act alone under their own power, advancing the organization's aims but taking total responsibility for their own actions and survival. These actions are chiefly focused on creating Black Rings and sabotaging the ability of DisCity's institutions to address the spread of Mania through the City-State.
The Underground's most senior known member is the Underground Shepherd, but it is not clear whether she directly leads the organization. Shalom theorizes that the reason why the Underground creates Black Rings is to nurture Great Existences for the express purpose of then feeding them to the Underground Shepherd.
In order to achieve their goals, agents of the Underground wield strange Mania-derived powers sometimes referred to as "sorcery". These powers frequently involve the conjuring of black flames, tentacles of sludge, and similarly ominous evocations. Field agents have displayed the ability to survive fatal wounds by reducing parts of or their entire bodies into sludge and reforming elsewhere. They frequently utilize inhuman puppets made from Mania contaminants, called Xenobodies, to act in their stead on the front lines of an operation. Perhaps the greatest power that the Underground possesses is the Shackles. Wielded by the Underground Shepherd (and perhaps others), the Shackles are referred to as the ultimate container for Mania and a pathway to the Source.