Possession
Many abstracts have the ability to possesses mortals - something they often “trade in” as they become more corporal, and can no longer move about the world of man with as much ease. It is usually impossible for an immortal to possesses an unwilling mortal, but those who’s souls have become so steeped in that which the Abstract was created from are often more then willing to be vessels, so much so that they may be possessed without strict consent.
Usually the possessed mortal becomes a strict agent of that abstract’s belief, continuing as he did before only with more power, and more fervor. Sometimes it is other abstracts that sneak into that mortals weakened soul, and guide him down paths he never would have found himself.
Possession can be a dangerous thing, however, for many abstracts that stay in the mortal body to long become addicted to it, and do not want to leave. In turn, many mortals do not wish to lose their new power. It is entirely possible, with the aide of the right immortal, for the two to merge together into one being. Technically a nephil, the merger is against immortal law.
Any mortal possessed for a certain period of time will “soak up” the residual power of the abstract possessing him, and become a mage, albeit a weaker mage.
Some immortals are also capable of possession, though it is a dangerous thing for them to attempt, as they must separate their core from their body, leaving them prone to the whims of those around them. Some might find it strange, but the abyss is most known for doing this, as apparently there has never been a case of another immortal for the abyss taking advantage of the vacant body of his fellows.