The world is comprised of two factions: Life and Death.
The Reapers serve Death while the Demons serve Life.
Demons are basically Rogue Reapers who refuse to continue the cycle of rebirth or simply want to protect one human in particular. Reapers are responsible for collecting souls, purging them of their identity, and bringing them to death so they can be reconstructed into a new living being.
Neither side is evil, they just have different opinions and conflicting views.
Life is like an overprotective mother who knows she must lose children in order to gain more. However she can't bring herself to harm them and focuses her attention solely on trying to lengthen the life-span of her Creations... But souls aren't meant to live forever. A soul grows weary with age and begins to rot the longer it remains untouched. Eventually, the soul will reach a point where It can no longer be consumed and reconstructed to form new life. This is how the first Reapers and Demons came to be made. Meaning for new creatures to be created, Life has to literally replace its' essence with her own in order for the cycle to continue.
Death, on the other hand, is focused mainly on preserving the sources they have available to them. His goal is to keep the cycle of rebirth going and make sure each soul is reconstructed in a timely manner. Mainly to prevent Life from destroying herself for the sake of their creations. For each time she has to replenish the stream, she slowly grows weaker. Eventually, her efforts will kill her and the world will become stagnant once more.
The humans play there own part. Being one of the last creatures Life and Death made together, Humans have sort of become the catalyst in the war between them. Very few humans know about the events that led to their creation but those that do are often considered to be either a Necromancers or Summoners. Summoners side with Death. Necromancers side with Life. Necromancers relish in longevity and have worked with demons for generations to help preserve the life around them. Summoners on the other hand, work with Death and aid in the recovery of tainted souls by opening forth portals for the Reapers to pass through. Knowledge of these two clans has slowly deteriorated over time but their teachings still remain, even if they are a bit hidden. Oddly enough, The Summoners tome has slowly devolved over time into what we know today as The Codex, while the Necromancy preaching have become a staple for The Holy Index..