Eye of God

The "Eye of God" is a seemingly omniscient 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 hands 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 and, as such, its original form greatly resembles it. Additionally, as a former part of the entity, Father has knowledge of its limits and the comprehension necessary to usurp it.

Trivia

 * There is a link between the entity and Truth. Truth might be its guardian, but 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 creature 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 creature, or the Gate itself, is presumably the source of all alchemy. When Edward sacrifices his ability to use alchemy, the Gate dissolves, severing his connection with the Truth and the being within the Gate.