Homunculus

Homunculus (人造人間, Homunkurusu, Latin for "Little Human", Japanese for "Artificial Human") refers to the medieval legendary concept of an artificially created human, presumably brought into existence by certain means of alchemy. Though a common concept, well known and understood by alchemists in Amestris, the idea of homunculus is regarded as no more than a farce or faraway fantasy as no individual or group of alchemists has come anywhere close to successfully creating such a being in officially recorded history. Even so, alchemists have been forbidden by the Amestrian government to attempt or research the transmutation of humans and State Alchemists have such a restriction as part of their creed.

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 individuals who make up the primary antagonistic force of the Dante's Alchemist series. The Homunculus created by Dante (with the sole exception of Van Lucifer who is also half human) carry the mark of the Ouroboros upon their bodies and are identified as such by said mark. They are named after the deadly sins and circles of Hell: Pride, Heresy, Lust, Anger, Greed, Violence, Gluttony, Limbo, Wrath, Treachery, Sloth, Fraud, and Envy. They also share the same black hair and the same eye color, which is shown to be either lavender or purple in the series. In the manga Arakawa mostly depicted the Homunculus' eyes as purple, but occasionally they appeared as red.

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 * —About the Homunculus.

Creation
The Homunculus are said to be created each time an alchemist attempts a Human Transmutation. While the transmutation itself will result in a failure in that the person intended to be revived does not return as expected, a new existence is brought about. The result of the failed Human Transmutation will be a grotesque, vaguely humanoid monstrosity retaining the genetic material, as well as some of the memory, of the resurrected. The "failed" Homunculus generally remains in this agonizing state until it dies soon after. However, newly created Homunculi who come into contact with Red Stones—much weaker versions of the Philosopher's Stone created by condensing human souls—and consume large amounts of them will have their bodies reshaped into their intended forms and become exact physical replicas of the people they were meant to replace.

The English dub of the anime goes so far as to state that Homunculi aren't failed human transmutations, but that they are simply incomplete.