When a Geist bonds itself with a dying person in order to create a Sin-Eater, the fresh created Bound receives a gift as a symbol of his new bond. This gift is a keystone memento — nothing less than the unbeating heart of the geist. It is a physical manifestation of the geist’s past and the key to its power. The act of offering this precious object to a Sin-Eater is a declaration of empowerment, the handshake of a partnership and, in part, a demonstration of submission and recognition of value. From this Keystone, the Threshold and respective Keys of the future Bound, as some skills that the Geist did once possess during his lifetime, stem.
Afterwards, a Keystone is almost universally stored in Twilight, protected by the respective Geist. If a Sin-Eater infuses the Keystone with Plasm, he can made it manifest, often to use it during Ceremonies. A Keystone can not be stolen or replaced, until their respective Geist is dead.