i absolutely love this concept and how he became a Beeg belly

This is the soul of a demon that is powerful enough to be materialized. It has a huge amount of energy, and the strength and weight of the victims possessed by it will be increased. The energy gained through eating when the victim is possessed will be absorbed by the devil. When the demon gains enough energy to materialize again, it leaves, but the victim's body will be permanently changed. Of course, if the devil's energy reserves are sufficient, even a brief possession by the devil will permanently change the victim.