Eye of God

A seemingly omnipotent being that exists on the other side of the Gate. The nature of the entity is never fully revealed, as only it's 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 heads.

Father was created from the entity's essence and has a bitter resentment towards it. He has knowledge of the entity's weaknesses and knows how to usurp it.

Father opens a gate to the being's dominion by harnessing the Earth's chi and is able to swallow the creature into himself, gaining it's abilities. 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

 * Father refers to the entity as 'God', though there is some debate about the authenticity of his claim.
 * There is a link between the entity and Truth. Truth might be it's 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 apperance and functions.
 * 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.