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 he has also identified himself as 'God'.
 * Unlike the Homunculi, who have simple purple eyes, the being's eye consists of five silver rings.
 * In the 2003 anime series, the being is 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.
 * The 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.
 * Through 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).
 * Since the Eye is the being who fashions objects whenever an alchemical reaction occurs, it can be argued that the Eye of God is a demiurge (an artisan-like figure responsible for the fashioning and maintenance of the physical universe). If this is the case, then the Eye is subordinate to the almighty, non-physical Truth.