A Corpus, also called a Deus Corpus or Ring Master, is a form of Great Existence capable of creating a Black Ring, and which serves as its owner once the Ring descends. The larval or nascent form of a Corpus, before it creates a Black Ring, is referred to as an Inheritance. Each Corpus possesses a core, a bubble of red flesh. Inside the core is a black hole, which contains a withered human figure, sometimes accompanied by the sound of a crying infant. Once it has created a Black Ring, a Corpus may create a new form for itself known as a Corpseborne out of the bodies and energies of humans and Corruptors in its area of contamination.
A Corpus is naturally attracted to huge sources of Mania, whether that be the strength of a powerful Sinner or the mass-Mania of large crowds. This Mania energy catalyzes a reaction which allows a Black Ring to erupt around it. This process may also require its host to have a "wish" or desire for the Corpus to grant. When a Black Ring appears, its Corpus seems to take on some properties of the individual or individuals who provided the Mania energy necessary for the reaction to occur. The wishes and desires of that person or people define the rules and laws of the world within the Ring, as well as (to some degree) influencing the personality of the Corpus. Thus, the interior of a Black Ring is the spiritual world of its host. While the Corpus controls this spiritual world, it is may only be borrowing the consciousness of the supplanted host, as Zoya was able to regain control of her spiritual world in BR-004 even while still within the Corpus's body.
Their inherent attraction to Mania also leads Corpuses to seek to devour each other. This reaction seems to be instinctual, and remains even after a Corpus has created its own Black Ring, suggesting that there may be a further possible evolutionary stage to their existence.
Most known Corpuses have been deliberately nurtured by the Underground, but Corpuses seem to be naturally occurring existences. At least one Black Ring, BR-001, was caused by a "wild Corpus" previously unknown to them. Although Corpuses are seem to be worshiped or venerated by the Underground to some degree, the amount of control that the Underground can exert over them is limited, as the Underground Shepherd becomes disappointed with the form and desires taken by Corpus 002. The Underground's true objective with Corpuses seems to be to nurture them until they reach a level of power suitable for them to be sacrificed and fed to the Shepherd.