Father

The creator of the Homunculi and the main antagonist of the manga series.

Xerxes
Father first existed as a being beyond the Doors of Truth, shadowy creatures with wicked grins and eyes that hold the ultimate knowledge of alchemy.

The Alchemist to the Emperor of Xerxes used his power to channel the being from the Gate, mixed it with his slave's, Number 23, blood and installed it in a glass container so it could survive in this world. Contrary to Pride's statement, Father could actually be considered the first Homunculus.

Being a homunculus with no form, Father was unable to perform any alchemy of his own, so he befriend the slave, taught him to read, write and perform alchemy and named him Van Hohenheim. The emperor called on Father (currently going by the name "Homunculus") to give him the secrets to become immortal. Father used this opportunity and instructed the Emperor to construct a giant transmutation circle out of the empire, creating bloodshed at specific nodes of the circle.

He then manipulated the event so he and Van Hohenheim were standing at the true center of the circle, so once it was activated, every soul in Xerxes besides Father and Hohenheim were sacrificed to possibly the first Philosopher's Stone.

Father used this power to open the Doors of Truth and created himself and Van Hohenheim new immortal bodies, living embodiments of the Philosopher's Stone with the millions of souls of Xerxes as their power.

Horrified at what he aided in doing, Van Hohenheim parts ways with Father.

The Sage of the East and the Founding of Amestris
After the destruction of Xerxes, Father made his way west where a small group of people were attempting to thrive. Father took these people under his wing and taught them alchemy, however he did not teach them true alchemy, but an alchemy that took its energy directly from the souls of Xerxes contained in Father himself.

From this point the people formed into a powerful military state called Amestris, a puppet government secretly run by Father. Amestris begins to rise in power and annexes all nearby land, eventually branching out to form yet another giant transmutation circle, with all skirmishes and bloodshed the Amestrian military taking place at the important nodes of the circle.

What Father plans to achieve by sacrificing Amestris as he did with Xerxes, but the transmutation circle is almost completed, so Father's plans may come to light very soon.