This creature doesn't have a form of its own, and looks like a Black Sphere when it first appears on the field. However, it quickly adopts the shape of the strongest monster in play, copying its respective ATK and DEF plus 1. The Avatar is at the same hierarchy level as "The Winged Dragon of Ra" and as such, it is unaffected by the effects of any other cards, including other god cards.
The names of this card's attack changes according to the monster that it is impersonating, basically by adding "Darkness" to the name of the attack of the impersonated monster. For example, when the Avatar took the form of "Slifer the Sky Dragon", its attack was called "Darkness Thunder Force" 「ダークネス・サンダー・フォース」.