From publisher blurb:
Oftentimes when a humanoid dies, their mortal soul passes through the veil into the next life, making the instinctual passage toward whatever afterlife has claimed that soul. However, every so often, a mortal’s soul lingers. Usually this is due to a strong will or unfinished business, and many of these souls becomes ghosts, spirits, phantoms - ethereal beings that plague the mortal realm until being vanquished by some adventurer who stumbles upon their eternal haunt.
But sometimes, these souls find new vessels, new bodies to inhabit. When a soul finds a living body, for all intents and purposes, whose soul has been wrenched out and left empty, a wayward soul with a strong enough will to return to the land of the living will merge with this body and once again find life. Babies born without souls, empty, uncrying. Empty corpses, souls torn away from them yet still have the ticking of involuntary mechanisms to maintain the very basics of life. These two types of empty vessels are sought out by these lingering souls and become permanently inhabited and become a Grim - a body possessed by an undead soul. However, a Grim is unlike any other living being, and stories of Grims are told throughout the world and are feared by many. However, many Grims are simply souls that wish to return to the pleasures and experience of life.