"Parasitism"
In essence, parasitism is about forming a symbiotic relationship with a target and gradually seizing control of the target's body.
After the Heavenly Demon Egg parasitized the sun, it continuously absorbed the sun's energy until it completely controlled and possessed the sun. By the time it successfully hatched, it would be a Heavenly Demon with stellar attributes—or rather, a star with Heavenly Demon traits—becoming an entity akin to an evil god.
Chapter 446