As a concept, Homunculi (Singular; Homunculus) are artificially created human beings made through either magical or alchemical means.
The reason the topic of homunculi is considered Black Knowledge is because the creation process is risky enough to the point where discussing them and inspiring people to attempt to create one could end in several deaths, if not hundreds.
The process requires several body parts from different individuals, the more individuals you have taken parts from, the better. Skin can be either patchwork or fully synthetic, and it is possible to use homunculi as a method to produce a patchwork of the greatest mages of our time.
The brain of a homonculus must either be created from scratch or purchased from a high-end dealer, as they cannot be substituted with a human or animal brain. Specific recipes for one can be found in the "Common Alchemical Components" article, with the specific workshops needed listed in the "Mage Workspaces" article. The commonly accepted reason that a homunculus cannot use a human or animal brain is that the brain will not accept a synthetic body comprised wholly of other people, even if reanimated using magical or alchemical means.
Though creating new individuals is fully possible, a common use for homunculi is to create body doubles for powerful, high-ranking mages. Another, less important use is to create mindless drones that perform manual labour in areas where spirits cannot be summoned or transported, and insider sources state that an estimated 1.2 million work in the Mage's Association to sort each and every file manually, as neither spirits nor undocumented mages are allowed to enter the buildings where official matters are handled.
It is currently estimated that 6 different homunculus prostitution and trafficking rings have been shut down, done so by tracking disappearances and murders following an MO identical in nature to Jack the Ripper.
While living with and maintaining a homunculus is not illegal, it must be evident that you perceive them as an equal to you. Any sign of a homunculus being mistreated or considered as lesser if it is not explicitly created as a body double or servant. An inspector is sent at random, yet regular intervals so as to make sure it is difficult to hide any poor treatment.
In addition to the usual human ailments, there are unique health problems a homunculus can fall victim to if it is improperly maintained via weekly blood plasma infusions and a healthy diet. These include the blood coagulating into pitch-black ichor, the eyes taking on strange colouration, and the skin going grey or peeling off in chunks. Other, less common problems involve physically melting, the complete and sudden disappearance of the immune system, and rampant mutation. If a homunculus is built well enough, it can survive and even become more powerful from the minor health issues such as the black ichor, the strange eyes, and the grey skin.
Addendum (0X-1X-2020); Researchers assisting in soul transplant experimentation involving Homunculi are required to wear full-head face coverings to prevent the newly born individual from forming additional, unhealthy bonds with nonessential personnel (Mandated in response to case R1-C-23022020)