Homunculi
Homunculi (ホムンクルス, Homunkurusu?) serve as the main antagonists in Fullmetal Alchemist. They are a group of artificially created humans who seek to fulfill their leader's goals of creating a Philosopher's Stone. Each homunculus is named after one of the seven deadly sins, and can be identified by their bearing of the mark of the Ouroboros somewhere on their body. They also possess a heightened regenerative ability, rendering them nigh indestructible, though not invincible. Aside from this, the concept of the homunculi varies drastically between the manga and first anime series.
In the manga, the homunculi were created by or are at the discretion of Father, being a manifestation of one of his traits: his greed, his lust, and so on. Father would later reveal that he ultimately created the homunculi for the sole purpose of purifying himself of his personal flaws. Each homunculus has a Philosopher's Stone somewhere in his or her body which fuels their regenerative ability, and were created at the same time as their respective Philosopher's Stone. While the first six were originally created from Father's own being, both Wrath and the second Greed are humans who accepted a Philosopher's Stone. The only way to kill a homunculus is by expending all of the power in their Philosopher's Stone, leaving them unable to revive themselves once they have been killed.
In the first anime, homunculi are the result of a human transmutation, and closely resemble the deceased human intended to be revived. These homunculi are led by Dante, and feed on incomplete Philosopher's Stones to fuel their powers; they are susceptible to death once these stones are regurgitated. The homunculi of the first anime possess an additional weakness: a remnant of their original bodies (a bone, hair, etc.), which weaken and immobilize them when they come in contact with them.ep.34
Read more about this topic: List Of Fullmetal Alchemist Characters, Antagonists