An Avatar is the mortal embodiment of a God or Goddess. In general, Avatars were born as mortals who devoted their lives to the service of their Diety. An Avatar may have been a great hero or villian, or a devout priest or infamous cultist, but in almost every case they were driven onward by their boundless devotion and faith. Avatars are used by the Gods to interact with the world of Terres, which is otherwise insulated from their influence by it’s unusually potent magical field.
At some point in their existence, each of these beings was approached by an Avatar of their God and given the opportunity to assume his or her role. Usually this is because the current Avatar has grown tired and desires rest, but sometimes it is because they have displeased their God and are no longer suited for the role. Whatever the case if the faithful person accepts, they are forever changed.
Each Avatar embodies one of the domains, or areas of influence of their God. Their form is exalted (or deformed) to display this association, and they become immortal and incorruptible vessels for their God. From that point onward, they share their bodies, their senses, their very lives with the God that they serve. At any time the God can choose to assume control of their body and express their will upon the world. Avatars are masters of the domain that they embody, although they rarely have significant abilities in any of the other domains their God influences.
Avatars tend to be somewhat distant and cold in the eyes of other mortals. They form temples, cults, priesthoods, and sequester themselves away from the world. Their mind is filled constantly with the presence of their deity and the endless flow of prayers that siphon through them and in to the divine realm. Often this is exactly what they have always desired, to be a testament to the glory of their God and an instrument of their divine will upon the world.
Rarely, Avatars become embittered by their service and use their might to spite the God who made them this way. Such rebellions, while terrible, are usually short lived as the God notices their wayward servant and controls them fully to begin the process of finding a new vessel. There are legends of evil or spiteful Gods forcing the role of Avatar on to the unwilling or bestowing the gift as a punishment. Such Gods usually force their Avatar to betray every person and principal they once held dear before stripping them of their powers and leaving them to face the repercussions. Thankfully such gruesome displays are mostly legend, but even the most agnostic person hesitates before truly committing a sacrilege.