A homunculus (ホムンクルス, homunkurusu?) is creature given birth through Alchemy by combining of human genetic material with several special substances and making the resulting embryo develop without the use of a womb. Though not confirmed, it is assumed that their body is made out of Ether (at the very least, Leysritt is). Incidentally, the term used to process of making a homunculus is “coining”.
The concept of “age” is meaningless for them, as they receive from nature all necessary knowledge and reasoning at the moment of birth. The maturing of their bodies into adult stage does not take years like humans, nor do they grow any older from that point on. As an example, despite their mature appearance and personalities, Leysritt and Sella have lived for little more than two years.
A perfect homunculus is pretty much a man-made artificial extension of nature that, external factors aside, will not die for as long the planet is healthy and has mana left. However, the price of being given life through such artificial means is that the homunculus will invariably have some sort of physical defect (small stature, short lifespan, lower intellect or lack of reproductive capabilities are just a few examples). Even with their potential longevity, they are below humans when it comes down to their strength as life-forms.
Despite having the same form and life than humans, homunculi are ultimately different type of existences for possessing powerful Magic Circuit. Regardless how much of a fragile life-forms they are, their Magic Circuits makes possible for the homunculus to achieve powers that are beyond humans. Fundamentally different than the magi that are humans with Magic Circuits, homunculi are better defined as “Magic Circuits with a human form”. Not “something made to be a magus”, but rather “something made to be a Magic Circuit”.
The homunculi created by the Einzbern, even those regarded as failures, have enough magical power to pose a threat to the Enforcers of the Association. The models based on Justeaze, such as Irisviel and Illya, share the memories and personality of the original. In a way, they're the same being. Illya herself is a very special case and her very existence can be considered a miracle in itself. She was “birthed” through conventional means by her mother, Irisviel, although she was put through several magical treatments while still in the womb. Said treatments made her body become even more unlike that of a human being than other homunculi and are supposedly the reason why her growth halted in pre-pubescent stage and why of her very limited lifespan (she is set to die roughly one year after the end of the Heaven’s Feel 5). Also, her knowledge of Magecraft seems to be something that she was born with, not something that she learned.
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