Xerxes

The Manga and 2009 series
Xerxes is an ancient country in the Fullmetal Alchemist universe. It was here that alchemy and Van Hohenheim have their roots. Hohenheim was a nameless slave, taught reading and basic arithmetic by a strange creature called Homunculus, which he had unknowingly helped to create with his own blood. Homunculus eventually convinced the king of Xerxes to build massive irrigation channels, which were actually a massive transmutation circle, in order to gain immortality. Immortality was gained, but only by Homunculus and Hohenheim, who was rendered a living philosopher's stone, a soul confined to a body made of other souls. The cost of this transmutation was the life of every one in Xerxes. Their bodies were later used to create Envy's true form. According to Hohenheim, he has 536,329 souls within him, making the number of people living in Xerxes 1,072,658 (not counting Hohenheim and Homunculus) and the time of its destruction.

The ruins of Xerxes are also used by a small number of Ishvalan refugees to hide in after the Ishval Civil War.

The 2003 Series
The nation of Xerxes is never mentioned in this series, however, the underground city below Central has some parallels to it. It served as a lost religious area when Izumi Curtis found a love letter from Van Hohenheim to Dante dating XX B.C. She later tells Edward that it was an abandoned time scale from the Christianity Era. (Discovered in the episode "Human Transmutation".) According to Dante, it was the birthplace of an ancient religion -- much like Xerxes -- which was wipped off the map. In actuality, it was sacrificed to make the first Philosopher's Stone by Dante and Van Hohenheim. The city was completely pulled underground using their alchemy to prevent discoveries of the Taboo. It serves as the main lair to the Homunculi.

Trivia

 * Xerxes was the name of the Emperor who ruled the Persian Empire from 485 to 465 BC. The country was named as a little easter egg from Arakawa, who also named the other central country of the series, Amestris, after his wife.