Sin-Eaters, also known as the Bound, are mortals who have died, but made what is referred to as the Bargain with strange, alien spirits called Geists to return to life. This does not come without a price; they must share their body, mind, and desires with the Geist, and often spend as much time confronting the death around them as well as appeasing their new "friend" as they try to continue their own life.
The Bound regain mystical power, or Plasm, depending on their archetype. They also must maintain Synergy, reflecting the balance of how well they work with their unearthly partner.
Geists are a special kind of spirit who are former ghosts that have separated themselves from their mortal identities, embracing the ideas and imagery of their deaths, coming to embody those images. They are capable of making special pacts with humans near death, combining with them to create the Bound.