Eye of God

The Eye of God (also called the Eye of Heaven) is a seemingly omnipotent 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 long black hands are shown of it. Whenever someone attempts human transmutation, it drags them through the gate and forces the truth into their minds.

Father was created from the entity's essence and has a bitter resentment towards it, but he has knowledge of the entity's weaknesses and knows how to usurp it. Father maneged to open a gate to the being's dominion by harnessing the Earth's chi. The entity attempt to decompose Father (presumably out of self-defense or simply instinct), but this backfired as Father was able to grab the creature by his "arms" and forcefully swallow it into himself, gaining its abilities, since the entity can be contained using a Philosopher's Stone formed from sufficient human souls.

When Ed punches a hole in Father's chest, the entity escapes and drags Father through the gate, absorbing him back into itself.

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 red 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 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.