Xerxes

The Manga and 2009 seris
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. Homonculus 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.

Religion
The underground city's was the only city in fullmetl alchemist that practiced christianity. (Discovered in the episode "Human Transmutation".) It was sacraficed to make the first Philosipher Stone by Dante and Van Hohenheim. The city was pulled underground to prevent discoveries of the Taboo. It serves as a lair to the Homunculi.