Tartarus is a tower consisting of six different blocks that erects from the depths of the ocean every midnight to the very west of Celestial Micronation's shoreline. Kirijo group's research justified that the tower is also known as the nest of hostile monsters called "shadows" who bear similar abilities as star children. Often, they would try to make their way out of Tartarus to swallow the entire humanity. Star children who are under a special contract and live in the dormitory are obliged to terminate these monsters, as well as exploring the depths of Tartarus in order to gain the truth behind its origin.