An Entity is a monstrous creation created by sacrificing the heart of a Rooder at her peak of power (age 15). The person who performs the ritual to become an Entity must be a blood relative of the Rooder whose heart they plan to sacrifice. Once the performer of the Ritual of Engagement takes in the heart of the Rooder (possibly eating the heart), they join to become a new Entity. It is never explained what an Entity looks like or shown, though it is in some sort of dark spectral form that is invisible to the living.
After its birth it seeks only one thing: to find a person with a weak heart. It then possesses the victim making him/her perform horrid acts of murder and violence. With every victim, the Entity becomes stronger. Upon the death of the victim, the Entity fuses completely with the soul of its murderous host and transforms into a Subordinate. From there on, the Subordinate continues its murderous rampage, gaining power with each life it takes. The only way to save the victim of the Entity is for a Rooder to destroy it as a Subordinate. Once an Entity is born, there is no way to stop it from taking over an unfortunate human's body and eventually becoming a Subordinate. All that can be done is for the Rooders to continue to fight the Subordinates and keep them from capturing young and unsuspecting Rooders before their power is awakened so they cannot be used in the Ritual of Engagement.