The Entity is an ancient beast that devours worlds across galaxies. She is the last boss of the Chaos Rings 3 game.
Upon realizing that the Entity was heading towards Marble Blue, people built a thousand ships within three years to head for other habitable planets. The 999th Ship, the Ark, was destroyed by the Entity, and the 1000th Ship, Theia, was stuck in orbit around Marble Blue.
The desperate humans who were left behind on Marble Blue discovered a way to use the power of the planet itself to fight back against the Entity. However, the planet was uncontrollable by their sacrifice, Elroux, and turned every human on Marble Blue into an Incarnatus or a Mutant.
It is discovered upon speaking to the yakoo on the World Tree that while the Child is the offspring of the Entity, the matured form of an Entity is the Qualia. Furthermore, Entities live in a colony around the Qualia and have a hive mentality, meaning if one of them is killed, the others would collectively strike back against the killer.