Arch-Enemy Event Weapon

The Arch-Enemy Event Weapons (事象兵器アークエネミー), also known simply as Arch-Enemies and the Event Weapons (事象兵器), are a set of eleven weapons, and the final stage of Magic Formula Armament. The first ten were tempered as a set of close range and melee-type weapons to directly oppose the Black Beast. An advanced tempering technique was needed to forge them, and a total of ten weapons were planned.

Information
The first nine were created during the First Great Magic War to combat the Black Beast. They were based on the Sankishin. Only those with the highest proficiency in the proper use of Magic Formula can wield them, and even then, there is a significant risk of mental burden on the user. It is said that the Arch-Enemy choose their masters. Just like magic attacks, they also cause scarring of the soul.

One of the primary functions of Arch-Enemy is to unburden their owners of "needless" emotions and enhance others; this has a profound effect, for example eliminating fear, but owners may also find themselves losing their compassion, remorse, etc. Strong-willed individuals such as Hakumen and Jūbei seem to be highly unaffected by this side-effect, however.

While being powerful enough to fight the Black Beast, Arch-Enemy are affected by the Boundary. If an Arch-Enemy with a released core is put near the Kiln, its ability cannot be activated.

The catalyst to smelt an Arch-Enemy are human souls. To create an Arch-Enemy is required smelting an Pseudo-Individuality (擬似人格) deep within the Boundary, it is able to observe itself and refine its existence. The stronger the observation, the stronger its value as a weapon it becomes. Depending on whether it takes the observation multiple or individual ways, is what determines its viability as a weapon. Observing its soul in multiple ways would make it more effective as a weapon, enabling it to make multiple observations simultaneously. The core's smelting process involves compressing the magic element (Fluid Magic Element Theory) into its threshold limit, forming a unit, and is then implanted with a "soul". In concept, this process is similar to smelting the core of an Event Weapon.

The core of an Arch-Enemy is the Black Beast itself. Back during the Great Dark War, Konoe A. Mercury had to create her own Black Beast, compress it, and seal its soul. She was able to do it by a magic spell, Infinite Gravity. The sealed Beast is compressed into the size of an apple. Then, the compressed Beast can be used as the catalyst for the newly created Arch-Enemy number 1, Highlander: Takemikazuchi. The core of Takemikazuchi was divided and used in the weapons number 2-9. The 10th Arch-Enemy, Divine Radiance: Murakumo, is unique in that it was produced after the First Great Magic War and it doesn't have Takemikazuchi's core.

There are also imitation Arch-Enemy called Event Weapon Imitations. They rely on atomic power instead of the life energy (human souls) used during the forging of the Arch-Enemy. They are replicas, but have comparable power and are recognized by the Arch-Enemy as being similar. Jūbei once experienced a resonance created by his Dream Blades: Musashi when it came into contact with one. There is an upgraded version of the Event Weapon Imitation known as the Idea Engine, but nothing else is known of it.

List of all known Arch-Enemy

 * 1) Highlander: Takemikazuchi (・タケミカヅチ) — A giant humanoid, who later was sealed within a satellite and sent away from the world. Eventually it was turned into the Embryo. Its core was used for the Arch-Enemy number 2-9.
 * 2) Ice Sword: Yukianesa (氷剣・ユキアネサ) — A katana with the blade made of ice.
 * 3) Demon Guns: Bolverk (魔銃・ベルヴェルク) — A pair of handguns with the power to pierce through space.
 * 4) Machine God: Nirvana (機神・ニルヴァーナ) — An automaton with a female appearance. Nirvana has her nails made of a material that negates Magic Formula.
 * 5) Dream Blades: Musashi (夢刀・六三四) — A pair of kodachis. Musashi's power is to cut "what cannot normally be cut".
 * 6) Slaying Demon: Ōkami (斬魔・鳴神) — A nodachi with the blade made of a metal that negates Magic Formula.
 * 7) Thundering Roar: Muchōrin (雷轟・無兆鈴) — A wand that appears to be as long as its wielder. Its abilities are the incarnation of matter.
 * 8) Snake Pair: Ouroboros (蛇双・ウロボロス) — A never-ending chain with a snake's head. It can cause direct damage to the victim's mind.
 * 9) Phoenix Wing: Rettenjō (鳳翼・烈天上) — A giant 55-inch nail. It is an Anti-Arch-Enemy weapon, and the core of Kushinada's Lynchpin, which has the ability to stop the functionality of magic element.
 * 10) Divine Radiance: Murakumo (神輝・ムラクモ) — A large sword which transforms into the armor and eight floating blades. This weapon was created after the Great Dark War, so it is the only Arch-Enemy without a part of Takemikazuchi's core.
 * 11) Corpse Burial: Requiem (骸葬・レクイエム) — A giant humanoid. It was made to replace the Amaterasu Unit, and create the new world that is not based on Amaterasu's dreams.

Trivia

 * This set of weapons is called in the Japanese version as Arch-Enemy Event Weapons. An "arch-enemy" is the principal enemy of someone or something.
 * This set of weapons is called in the English version as Nox Nyctores Causality Weapons. "Nox Nyctores" is a term made of three words: "nox" – Latin word for night, "nycto" – Greek word for night, "res" – Latin word for thing.