Eye of God

The "Eye of God" is a being that exists on the other side of the Gate. The nature of the entity is never fully revealed, as only its eye and a series of tendril-like black arms are ever shown. Whenever someone attempts Human Transmutation, it drags them through the Gate and forces the truth into their minds.

The Homunculus later known as Father was created from the entity's essence.

Trivia

 * There is a link between the entity and Truth. There is a possibility that they are one and the same, as Truth has also identified itself as 'God'.
 * In the 2003 anime series, the eye exists however it's mostly replaced by The Gate Children in terms of appearance and functions.
 * In the 2009 anime, when Father is dragged through the Gate one can hear multiple voices laughing, this is possibly a reference to The Gate Children of the first series, however just like in the manga it appears to be only one entity.
 * In the manga and the 2009 anime the eye appears in every scene involving the gate opening or human transmutation, however In the 2003 anime the eye only appears in 2 scenes, serving the same role:
 * When Izumi Curtis offers her son to the gate
 * When Dante opens the gate to Hohenheim of Light.Eye2003-1.jpg
 * A characteristic exclusive to the 2009 anime occurs when human transmutation is activated, where the eye opens in the ground on top of the transmutation circle.
 * This can be seen when the Elric brothers tried to transmutate Trisha, when Izumi Curtis try to transmutate her child and when Father made the philosopher's stone out of the people of Xerxes.
 * The color of the eye varies in the 3 adaptations:
 * In the manga the eye is a light pinkish shade of red, probably to match the eyes of the homunculi.
 * In the 2003 anime the eye is a shade of purple, matching the eyes of the homunculi and the Gate Children.
 * In the 2009 anime the eye is grey.
 * Due to that change in the 2009 anime, the eye color only matches the homunculi/father eye color in the manga and the 2003 series.
 * Eye2009-1.jpg being, or the Gate itself, is presumably the source of all alchemy. When Edward sacrifices his ability to use alchemy, Edward's Gate dissolves severing his connection with the Truth and the being within the Gate,
 * Eyemanga-3.jpggh Father's dialogue to his sacrifices before achieving his purpose, he asks them if they ever considered the possibility that "the world they live in is a massive organism", and goes further describing it as a "neural system". This seems to hint at the nature of this entity's function, as a metaphysical neurological system storing all information in the universe, which may further be implied by the eyes and synapse-like hands. It may also be a reference to the Gaea hypothesis, which suggests that the biosphere (the totality of life) of the Earth itself should be considered a life form ("Gaea" is the name of the personification of the earth in Greek Mythology).
 * Some fans have noticed the possible connection with the eye's appearance and some illustrations of the Ein Sof, something not far-fetched with the kabbalistic allegory in Fullmetal Alchemist and the role represented by the eye itself.