Though anyone can be possessed by the souls of the dead, it is more common among sorcerers than those without magic, and it is most common among Asirim. It is the decision of the soul to possess an individual, but Asirim can summon souls to ask for aid in certain activities. Souls can grant Asirim greater power for spells, and souls can use the bodies of anyone, not just Asirim, for their own ends.
When souls possess bodies for their own ends, and not through a summoning, they have never been known to do harm to the person they possess, or to anyone else. The souls of the dead seem to only want to do good, or at least to do no lasting harm, in the world of the living. These souls have been known to kill while using an individual’s body, but everyone who has been killed in such a way was known to be cruel or a killer of innocents. These possessions are acts of protection, saving the one possessed and those in his or her life, from the one who wishes them harm.
Possessions are very rare among those who are not Asirim or the non-magical children of Asirim. There are stories of souls saving entire kingdoms by selecting a hero to work through, but in most cases these heroes are Asirim or their children.