Ars Armagus (術式兵装 Jutsushiki Heisō, Technique Formula Armament) is a term that has two meanings.
- It is a more powerful form of Ars Magus that is used with the intent to kill one's opponent and requires a tremendous amount of seithr, as well as a high level of Ars Magus aptitude. During the First War of Ars Magus, it was used to fight in close proximity to the Black Beast, and consumed seithr which came from it. Like special weapons based on it, Nox Nyctores Causality Weapons, Ars Armagus has various negative side effects, which include abnormalities in souls and emotional numbness.
- It is a type of weapon that was made with the use of Ars Magus, and is worn on the body. One of the most known of these types of weapons was the Sealed Weapon: Izayoi. Many of the Izayoi were produced but only one or two remain today.