Possession is the power for one being, to posses another and take control over them. This is a rare power, and very few creatures possess it.
When possessing another, they gain all the powers that that being possess, as well as there own powers.
Seers are more resilient to this power than other beings.
Beings with this power
Demons - All demons possess this power, and can possess nearly anything.
Tunguuk's mask - Anyone who wore it was possessed by the mask's power.