Nationwide Transmutation Circle

The Nationwide Transmutation Circle, as it is most often referred to, is a gigantic Transmutation Circle being dug in the form of an interconnecting underground tunnel series all around the nation of Amestris. By the shape that the massive array takes, it appears to be the mysterious double pentagon circle that was first discovered by Edward Elric in Laboratory 5 - the Transmutation Circle designed to create a Philosopher's Stone. If activated, the Nationwide Transmutation Circle will consume the lives of every human being in the country and fuse them into an enormous Stone to be used by the mysterious being known as Father.

Just as a normal-sized Philosopher's Stone Transmutation Circle is powered by a number of live humans placed at multiple intersecting points, the Nationwide Transmutation Circle is designed around ten locations near the outskirts of Amestris. These locations have, as part of their preparation for the circle's activation, been orchestrated as places of extreme bloodshed in the form of civil wars, rebellions-turned-massacres and grisly border struggles in a ritual called the Crest of Blood - in which the lifeblood and sorrow of the people is soaked into the land, making it ripe for the harvesting of souls for the Philosopher's Stone. These ten points of intersection (Riviere, Cameron, Fisk, Wellesley, South City, Ishval, Fotset, Pendleton, Reole and Briggs) will serve as activation points while all humans within the circle become materials for the transmutation.

While Dr. Tim Marcoh, the Crystal Alchemist, was among the first to realize that the Homunculi's plan was to create an extremely powerful Philosopher's Stone using the people of Amestris, it would be up to Maes Hughes to discover the true secret behind secrets - that the nation of Amestris had been built from scratch specifically for the purpose of constructing and activating the Nationwide Transmutation Circle.

Purpose
The only known benefit from a Nationwide Transmutation Circle is the immediate creation of a perfect Philosopher's Stone. The many souls from the victims within the Circle are sacrificed and transferred into one stone or even into the body of one individual. Those standing at the very center of the Circle are the only ones impervious, instead, becoming immortal as a drawback from becoming human-formed Philosopher's Stones.

Xerxes
In the case of Xerxes, the Nationwide Transmutation Circle was dug out unknowingly by the citizens of Xerxes, believing the King was making them an aquaduct. Father, in his plan to gain a physical form, persuaded the King of Xerxes into creating a Nationwide Circle and massacring villages in certain places around the kingdom all in the ruse of making the King immortal. Once the Circle was completed, Father had a young Van Hohenheim stand with him at the center during activation, while all the thousands around them instantly perished.

Amestris
Once Hohenheim had run away to Xing, Father created a small new settling to the west of the ruins of Xerxes called Amestris. The ultimate goal of this new place was to create another Nationwide Transmutation Circle around it. Once Father's alchemy had been passed on to the early inhabitants of his territory, he vanished into obscurity, allowing it to grow into a fully-functioning country. Once established in his lair underneath Central City, he had his Homunculi carry on specific tasks to complete the Circle.

Sloth: Tasked with physically digging the underground tunnel around the entire country with his hands. It is implied he has been doing this job for at least a century.

Pride: Personal enforcer of Sloth. With his shadow powers, he is also responsible for keeping the tunnels safe from any intruders.

Wrath: He was to be the Führer of Amestris and the leader of the Military. With his ruthless imperialistic position, he could easily spread the bordering massacres needed for the Circle's completion. The infamous Ishvalan War was one of these.

Lust: Responsible for being the indirect catalyst in many of these massacres. As an instigator of sorts, she would spread lies or false promises in the hopes of creating large conflicts that would lead to bloodshed.

Envy: Much like Lust, he was a major instigator in many of these disputes. He was directly responsible for starting the Ishvalan conflict when he shot a child in the guise of an Amestrian soldier.

Activation
In Chapter 104 of the manga, the Amestrian Nationwide Transmutation Circle is shown to be finally activated by Father. Once done so, the transmutation begins to drain everyone around the country of their souls. The chosen few at the very center of the circle (Edward Elric, Alphonse Elric, Greed/Ling Yao, Scar, Mei Chan, Van Hohenheim, Izumi Curtis, Tim Marcoh, and Roy Mustang) are all rewarded immortal bodies, while Father now possesses "the strength of God" within him. He is also shown to be the host of a new form for himself, one which resembles a young Van Hohenheim.