An Incarnation is a sapient embodiment of a concept, a monster in the machine as referred to by others. It is an idea given life and thus unkillable with weapons. It is supported and fed through certain ideas, and you defeat it by taking the strength from that idea. Most often, people simply accommodate them.
An incarnation comes into being when a person summons and meld into it and if there are multple copies wandering around they may absorb the lesser versions for strength. They cannot go against their nature as incarnations, which makes them somewhat predictable.