The human body contains mana, the life force that can be transferred for use as a source for magic. The individual's capacity for mana slowly grows with time as the person ages into adulthood, finally reaching its limits during middle age, and then steadily declines. A mage must take care to control the amount of mana they utilize, as using too much at one time can sap their life force and lead to their death. However, a mage can substitute the mana of others for a magical source with no cost to themselves, transferring all the risk to the victim and leaving their own mana intact for use in less risky endeavors. The Incan empire is a brutal regime that is built upon magecraft, the use of magic to shape the natural world. It expands its territory through the conquest of its neighbors, taking captives as slaves to use as fuel for their rituals. These victims are ritually sacrificed on an altar by Incan priests, who use the mana released from the death of the victim as fuel to power their spells. However, this has the potential of backfiring. This system creates many enemies among Incan neighbors, who may band together or fight to the death against them, knowing what will happen to their people. It also can lead to slave revolts among captives. Luckily, magecraft seemingly provides an alternative solution.
Within each male sperm cell is a microscopic organism known as animalcule, a complete preformed individual representing miniature versions of human beings. These preformed humans develop and enlarge into fully formed human beings through the process of conception and birth. Magecraft allows individuals to bypass this long and convoluted process to create life in order to create a perfect servant loyal to its creator, known as a homunculus. These homunculi are grown within a specially built cauldron designed to hold magic brews. This brew is filled with various ingredients, such as eye of newt, as well as other lay ingredients, such as cow intestines and the "seed" of a male. The resulting "child" emerges from this concoction as a fully grown adult, bound to obey its master's commands. Although they are intelligent, homunculi lack free will and individuality, making them the perfect servant.
The Incan empire have considered the potential of swapping out captives for homunculi for the purpose of using them in their rituals. On the surface, the benefits are obvious. Creating a literal slave race bound to your will would make methods of control much easier and cheaper, mitigating risks. As they are created from magic as adults, they contain all the mana they need at birth without having to go through the long timeframe of aging to an appropriate level, saving time. In addition, they can be grown in bulk, as the ingredients aren't exactly rare. As such, the theory of replacing captives with homunculi in the flesh economy is sound in theory. What would preven this system from working?