Necromancy magecraft. The style of magecraft that Shishigou Kairi uses.
Necromancy falls under the magecraft systems of both communicating with the dead and curses among the departments within the Mage’s Association, and the term specifies all magecraft styles that primarily use and process corpses. Even after the soul has vanished, a magus’ body continues to have extremely potent magical properties, so the corpses of magi are more useful than regular corpses. Besides magi, corpses of things like beasts with magical properties are also highly valued by necromancers. For instance, a Phantasmal Species can be made into a suitably strong Mystic Code. Shishigou Kairi processes pieces of corpses such as these for every use he could. For example, he used things like monkey hands, owl eyes, and the hearts of magi.
Shishigou Kairi’s Mystic Codes were very high quality, and most of all were easy to use since the Mystic Codes that he made from corpses lacked negative factors such as “regret” and “malice” attached to them (while these increase a Mystic Code’s power, it proportionally raises the possibility of it going berserk). And those who should have fallen into ruin ended up becoming the subject of conversation among magi again—that includes the story of how Shishigou Kairi’s Magic Crest was turned poisonous, though.

Fate/Apocrypha material: Fate/Apocrypha Encylopedia