At the very beginning of BLEACH, the Shinigami Rukia Kuchiki explains the basic structure of the series' cosmology, delving into the reason entities like herself exist, and the balancing role that the Shinigami play in maintaining the universe's current form. Not much later, during the Substitute Shinigami arc, the series introduces the Quincies and their near annihilation at the hands of the Shinigami.