However, off the official record, not only one, but several of these creatures have been created in arcane, sinister secrecy. In the most relevant sense of the word, "Homunculus" refers to the eight individuals who make up the main antagonistic group of the Fullmetal Alchemist series. These created humans, named for the seven deadly sins (Pride, Lust, Greed, Envy, Sloth, Gluttony and Wrath), along with their Father, are beings with incredible longevity, astonishing regenerative properties and a host of varied superhuman abilities. The Homunculi usually carry the mark of the Ouroboros upon their bodies and are identified as such by said mark.
The seven Homunculi are the spawn of Father (who himself was the earliest known Homunculus, birthed inside a flask in the ancient country of Xerxes). After having used half the population of Xerxes in the creation of his body, Father himself became a Philosopher's Stone of sorts and became able to siphon off smaller stones from his body to create minions he calls his children. Each Homunculus, as a former part of Father's body and consciousness, is said to have been made up of a different emotional vice - corresponding to their name - and carries that vice as a part of their personality or identity (with Greed being avaricious, Pride being arrogant, Wrath being irascible, and so on).
These Homunculi have physical compositions similar to those of humans but consider themselves superior due to their heightened strengths and abilities. They boast human intelligence and emotions, taking pride in their existence as higher beings, but feeling humility and gratitude toward Father for having given them life. As a result, these Homunculi are largely devoted to the completion of their Father's grand plan (with the exception of Greed, as he defects twice). Each of the seven Homunculi has a Philosopher's Stone as its core, fuelling its life, regenerative ability and other abilities with a multitude of souls. Each Homunculus can be damaged to the point of death, even to the point of having his or her entire body disintegrated, but can only truly be killed when the Philosopher's Stone heart is either destroyed or runs out of lives to use as power. Envy and Pride have proven capable of absorbing the lives from other Homunculi into themselves to lengthen their lifespans, but it is unknown whether or not this is a trait shared between all seven.
Though the original six Homunculi have bodies that were built around the Philosopher's Stone, for the sake of his machinations, Father began experimenting with a new method of Homunculus creation. A new set is created by injecting one of Father's stones into the body of a living human being. The souls in the stone and the soul in the body clash for supremacy, often destroying the body entirely, until one consciousness takes control. Homunculi created in this manner are able to age - a property of which their Stone-based brethren are incapable.
As of now Father has been creating a whole secret army of Homunculus one by one. He has been creating these Homunculus in a secret facility somewhere in Amestris. Some stay to fight for Father while others follow their beliefs and fight against father.