Soulslavery, sometimes euphemistically called soulbinding, is the ability to control other souls by means of one's own soul. If a soulmaster is controlling more than about nine people, or is controlling less than that over a distance surpassing a kilometer, then soulslavery begins to require absolutely enormous amounts of energy. There are two ways this energy could be obtained: 1. feeding that amount of energy to the soulmaster (i.e. funneling energy into them slightly faster than it is pulled away from them, so as to prevent them from exploding or burning alive), or 2. having a soulmaster that inherently has that much energy (i.e. they are a god of destruction, powerful psychic, nameless child, etc,).