Human souls are immortal, and possess the potential to become one of two things after they die. Their souls can rise to the celestial realm as part of the celestial host, becoming infused with the realm's energies and transforming into an angel. Alternatively, their souls can descend to either the Abyss, a realm of demons, or the 9 Hells, the realm of devils, where they will be fought over by both sides. Over thousands of years, the souls will be brutally tortured for their energy to serve as fuel for the denizens of the Abyss or the 9 hells until the soul fades into oblivion. Demons have the ability to possess human mortals, transforming them into demonic hosts and giving them the powers of that demon, as well as enhancing physical attributes such as strength and speed. Through a summoning ritual, they are able to forcefully take control of a human and cause havoc on the plane, using the host as a puppet. The devils also wish to use this ability to take control of human hosts in order to walk the earth, but there are significant barriers to this plan.
Devils are former angels who rebelled against God in the war of heaven. These rebels, led by Asmodeus, were cast out of the celestial realm into hell, where they organized their own hierarchy and made war with the denizens of the abyss. As angelic spirits are basically souls super charged with celestial energy, they should be directly compatible with human souls. This should make humans a perfect glove for the angel, allowing them to a perfect subject for possession. However, this is not the case. The devils are forced to take alternative methods to complete their plans. This is done through the creation of cults devoted to worship of these devils. These cults would undergo a breeding program over the course of hundreds of thousands of years to breed an artificial soul, known as a homunculus. This soul would be able to be possessed by Asmodeus, and allow him to walk the earth and conquer it.
The human soul was specifically designed to absorb celestial energy, as they were meant to become angels. As such, they should serve as perfect hosts . It is not a question of free will, as Asmodeus and his legion were capable of rebelling, proving that angels were not slaves to God's machinations. Nor is it a question of power, as there are demons who are as powerful as devils, with some even surpassing them. What would prevent human hosts from being perfect vessels for angelic spirits? A good answer would center around how an artificial soul would be a better fit for possession than a naturally created one.