The main protagonist of Soul Sacrifice is the slave that serves a powerful and cruel sorcerer. Just before the protagonist is going to be sacrificed, a demon in the form of a book appears before him. The book is a collection of stories that describe past fights between monsters and the powerful sorcerer. The player character is able to enter the book's world and experience the fights, thereby gaining experience and power.
The core mechanic of the game is the ability to sacrifice parts of the character's body or items to create devastating attacks. These sacrifices will be permanently marked on the player character's body, meaning that they are not an infinite resource that can be tapped into.
Source: Wikipedia, "Soul Sacrifice (video game)", available under the CC-BY-SA License.