An Avatar is a creature composed of and created by a Deity, typically taking the form of a human or other sentient race, though they can take any form, as long as they have physical substance. Avatars are direct physical representations of their Deity. All Avatars of the same Deity are equally a part of that Deity; they can no more keep secrets from each other than a person's left hand can keep a secret from his right.
As they are made of Deific Energy, they can not be harmed by ordinary weapons. Only magically enchanted weapons can do damage to an Avatar. Magical spells of any kind can also harm an Avatar, though Avatars are always immune to magical energy of the type that sustains them. For example, a Deity who places the sphere of Cold in an Avatar is immune to any cold-based attack. This does extend to weapons, so that an Avatar of Weaponry is completely immune to physical weapons of any kind. Unless harmed with orichalcium, the destruction of an Avatar has no particular impact on the Deity that created it, and the Deific Energy used to create the Avatar can be re-used immediately.
A Deity can create an Avatar in any form, from a wisp of smoke to a rock, but they almost entirely look like an ordinary person of some race, but with symbols or instances of their respective Zones. Avatars can not interact with mortals in ordinary ways: they are simply too awe-inspiring, fearsome or otherwise overwhelming for any mortal mind. In this way, an Avatar is forced to interact through symbols, riddles and similar abstract ideas that will stick in mortal's minds and that they will decipher later, when they regain control of their mind. Avatars therefore can not directly order an army around, or handle the day-to-day affairs of a kingdom - they simply can not do everything themselves, but they also can not effectively give orders to underlings.
In most cases, no matter what they look like, an Avatar is obviously distinct from other entities in widely-varying ways. A goddess of beauty is likely to be phenomenally beautiful, for example, while a god of cruelty will inspire a sense of impalpable and perhaps subtle dread. An Avatar who looks like an ordinary human could walk through a crowded marketplace, and there would be no obvious change to the crowd, but if one were to ask the next day, every single person that had the opportunity to see the Avatar noticed and remembered him. The only exception is a Deity whose Zones include something like Subterfuge or Camouflage, who have the same impact on any person who does see them, even if they also have powerful abilities to remain undetected.