The Pledge of Immortality is a deal that can be made between a human and a mazoku to confer partial immortality. When the human makes the deal, their soul is stored inside of a Pledge Stone. As long as the stone exists, they cease aging and recover instantly from any injuries suffered. They can only die under three conditions: if the Pledge Stone is destroyed, if the mazoku with whom they made the Pledge is destroyed, or if they are hit by a spell calling power from a being more powerful than the mazoku with whom they made the Pledge. (It is somewhat unclear whether only black magic spells can be used for this purpose.)