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 based on the Sankishin, and they are the final stage of Magic Formula Armament. They take the form of Grimoires converted into weapons, smelted for fighting the Black Beast.

Overview
Smelting the Event Weapons requires extremely advanced techniques, and only ten were planned in total. The first nine were created during the Great Dark War by Nine of the Six Heroes to combat the Black Beast, and the war ended before the tenth weapon, Divine Radiance: Murakumo was completed. Several of the Event Weapons were lost during the war. The eleventh, unplanned weapon, Corpse Burial: Requiem, was secretly in development after Nine learned the truth about the world.

Just like magic attacks, they also cause scarring of the soul.

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. A finished Arch-Enemy has its own will, chooses its master, and attempts to protect its user.

One of the primary functions of an Event Weapon 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, the Arch-Enemies are affected by the Boundary. If an Arch-Enemy with a released core is put near the Kiln, its ability cannot be activated.

Creation
Creating the Event Weapons requires a large number of souls as well, in other words, humans. To create an Arch-Enemy, required is smelting a 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".

The core of an Arch-Enemy is the Black Beast itself. Back during the Great Dark War, Konoe Ayatsuki 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 could be used as the catalyst for a Legacy Weapon, now also an Event Weapon number 1, Highlander: Takemikazuchi. The core of Takemikazuchi was divided and its parts are used in the weapons number 2-9. The 10th Event Weapon, Divine Radiance: Murakumo, is unique in that it was completed after the First Great Magic War and it doesn't have Takemikazuchi's core.

Imitations
There are also Event Weapon Imitations. They rely on atomic power instead of the life energy (human souls) used during the forging of the Arch-Enemies. They are replicas, but have comparable power and are recognized by the Arch-Enemies 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.

All Event Weapons

 * 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 Event Weapons 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.