An artificial life-form made with alchemy. The ones that Yggdmillennia created as part of their army and as sources of mana supply were made using the techniques of the Einzberns, one of the Three Families who held the ritual of the Holy Grail War in Fuyuki. Einzbern homunculi are made using Magic Circuits as their foundation; to put it another way, they are like deteriorated spirits which were spontaneously produced. As a result, they are extremely useful as mana supply batteries.
However, unlike those made by the Einzberns, which are like works of art, the ones made by Yggdmillennia are a mass-produced product made in large numbers. They are inconsistent in their respective quality, but on the other hand this aspect expresses itself in the homunculi who have some slight individuality. Sieg was a mana supply homunculus and at first couldn't even walk properly. Mana supply homunculi were made with the concept that they just need to produce mana, so many of them very defective as living beings, such as not being able to breathe from the time they were born.
The one put in charge of making them was the most prominent alchemist within Yggdmillennia, Gordes Musik Yggdmillennia… He's no good at fighting, but he's actually quite capable when it comes to alchemy.

Fate/Apocrypha material: Fate/Apocrypha Encylopedia