The Homunculi are creatures created by using alchemy on the cells of an already living creature and injecting the cells into a human body. This process is made possible through the use of carrier embryos which enter the body and make their way to the brain, where they turn the human they have infected into a Homunculus. If the embryo is implanted directly into the brain, then the transformation will be almost instantaneous and almost painless, but if the embryo were to be injected at any other part of the body, it will cause severe pain to the person as it makes its way up to the brain. After the creature has become a homunculus, they gain many different powers, including invulnerability, healing, immortality, super strength, super speed, and a need to eat humans.
There are three different types of Homunculus:
The Animal-Type Homunculus is the weaker of the two types, resulting when the carrier embryo is implanted with the cells of an animal and implanted into a human. When this happens, the human will be capable of transforming into the form of the animal from which the cells were taken, as well as being able to transform into a human. This transformation is severely harmful to the human host though, causing them to die, and their brain to be taken over by the experiences and instincts of the donor animal. The Animal-Type Homunculi become amazingly loyal to the human that created them, much as a pet becomes loyal to its owner, though some seem to be less loyal than others. Their power is entirely dependent on their animal and on their own will (as demonstrated by Kawazui’s being beaten by Kazuki Muto, but Washio being able to battle both Kazuki and Tokiko on equal terms.)
The Plant-Type Homunculi occur when the cells of a plant are placed into the carrier embryo. The Plant Type Homunculi are very similar to the Animal-Type Homunculi, as both have both a human and a “natural” form, and both the Animal and Plant-Types’ power is entirely based on their own strength and determination. The Plant-Type Homunculus are also arguably the easiest type of Homunculus to make, as Papillon’s first successful Homunculus was a Plant-Type, but this may also have only been a coincidence.
The Human-Type Homunculi are the most powerful type of Homunculi, due to their ability to use the Kakugane, an ability reserved for humans. The Human-Type Homunculi are created when a Human uses his own cells in the carrier embryo, and he would then implant this embryo into their own body. This causes them to gain all the powers of a Homunculus without their minds being destroyed. In other words, they are able to keep their memories, their thought processes, and their ambitions.