Given the major roles that ghosts and the undead play in The Caged Underworld, death is a major component in the story. In-canon, death is established as initially being a force sealed in the then-empty realm of purgatory. After Pluto's assisted suicide and descent into purgatory, he was introduced to these forces unfiltered, which contaiminates and fused with his soul to the point where his very prescence prompted death in living species. This is shown after his brief escape back to Earth, unintentionally spreading death with him like a worldwide virus to the previously immortal human race.
Death in Humans
In-universe, when humans die, the soul remains inside their body in a state of sealed stasis until reaped by a ghost. Here, the deceased person, although void of life, experiences an oneiric, vaguely comatose existence inside the Limbus Realm, a structure created from the person's dreams or objects of happiness. When the soul is reaped and the seal chaining them to the Limbus Realm is removed, they awake from their fantasies, triggering the appearance of an anobaith, which the reaping ghost has to destroy using his or her unique abilities bestowed upon him or her in order to access the human soul. In exceptional cases, the anobaith can return into the respective corpse's body, resulting in a near-death experience; however, this does not apply if the corpse has experienced advanced decay or injury.
Death in Ghosts and Priests
Despite having already died a human death, it is entirely possible for ghosts and priests alike to die a second time due to a variety of reasons. When a priest dies, he or she does not relocate to the afterlife, as this is a trade-off when they regained their human bodies. Unlike normal humans, ghosts and priests have a much higher resistance to injury; thus the process of death takes much longer than it would in comparison with a normal human.
Absolute Death is a phenomenon that occurs only in ghosts (or, in the case of priests, when they have been in Pluto and Persephone's presence for an extended period of time). Absolute Death can happen as a result of a ghost being killed, ectoplasm withdrawal symptoms, or by overusing their powers or letting them grow to the point where they become unstable and literally consume the ghost (the latter of which can be caused by being with Pluto and Persephone for too long, as being exposed to their death-infused souls can accelerate the process). The difference between death and Absolute Death is that if a ghost or priest succumbs to this, he or she relocates to neither the afterlife or to the river Styx, and he or she literally ceases to exist.