A Magic Circuit (魔術回路 Majutsu Kairo) is vital to make an Automaton functional.
A Magic Circuit is the most vital mechanical device in any Automaton, because it provides functionality, be it basic living skills or combat powers. In theory, a Magic Circuit can function and subsequently, run complicated Magic-related processes on its own. However in reality, this is not entirely true as the Magic Circuit needs and relies heavily on Mana in the first place, in order to even show the effects and outcomes of its processes. Without any Mana, a Magic Circuit cannot function. Additionally, how powerful an Automaton (and its Magic Circuit) is relies on the Puppeteer's abilities and talent to control it.
A good analogy of a Magic Circuit is a passive engine that has the potential to make a (train) locomotive fast and powerful. However, by itself, it does not work. Thus in order to make it run, it must be in contact with pressurized steam power that will make its gears turn. Once this happens, the engine becomes active and can function as how it is meant to. Then, how fast a locomotive can go depends on how well a train driver controls the engine room on board. Likewise, once a Circuit comes into contact with Mana, it activates and becomes fully functional. Its performance level is then dependent on its Puppeteer's skills.
There are many variants of Magic Circuits, differentiating from name and abilities.
- Heart of Eve (or Eve's Heart) (イブの心臓 Ibu no Shinzō): This is primarily most important Magic Circuit, for which it symbolizes life; every Automaton is equipped with it and it is widely used. This Magic Circuit is responsible in allowing an Automaton to act independently and provide intelligence to them as well; in the hands of a skilled Puppeteer, an Automaton can be made to look and act similarly to humans. If the Magic Circuit were to disappear or be removed entirely by any means, it guarantees death to an Automaton.
- Kongouriki (
金剛力Kongōriki, lit. Herculean Strength): This Magic Circuit will grant an Automaton an unrivaled, peerless strength. It has the ability to increase muscle strength by a factor of several thousandfolds, by changing monads in its own territory to super hard material. As a result, a user can remarkably improve his or her offensive strength in attacks and defense. This allows the user to demonstrate incomparable strength in close combat. The hardening also extends to the blood as well. Though it offers superb offense and defense, it should be noted that this Magic Circuit has its weaknesses as well. Shoko Karyusai reveals that this Magic Circuit cannot go against the laws of the world. Also, if the owner of this Magic Circuit was to be hit by something related to sorcery, such as the spells of Charlotte Belew's Automaton, the owner would be annihilated. The defense is not absolute as well, as there is an exception that can cut through the owner's hard armor. Despite of all the good support this Magic Circuit provides, Raishin Akabane calls it simple. This Magic Circuit is currently owned by Yaya.
- Gram (
魔剣Guramu): As pointed out by Shoko, it revolves around the usage of sorcery, and anything that has a form would be destroyed. It has the ability to preside over the creation and destruction of matter, and it can decide if matter exists or not. It extinguishes matter by the element of extinction. In addition, an Automaton that has this Magic Circuit can increase its own mass. It is currently equipped on Sigmund.
- White Mist (
白い幻霧Howaito Misuto): The Magic Circuit that grants the Automaton to turn into a white mist of vaporized elixir, an aqueous solution and corrodes everything it has contact with. According to Felix Kingsfort, no matter how strong an opponent's ability is, the attacker cannot assault something as shapeless as fluid. Even though it is seemingly impossible to counter the ability, it is actually possible by adding a different kind of sorcery in the Mist, which will cancel its perfection. It was installed into Snow White.
- Predator (
魔術喰いPuredetā): A Magic Circuit that allows the usage of multiple kinds of Magic after obtaining a Magic Circuit from another Automaton. The specified number of Magic Circuits it can collect is not properly specified, but it managed to reach up to 20 Circuits. While it is reasonable powerful at its own might, it has its own weakness too: the previous Magic that was used before can only be used once and the Magic Circuit that contains the previously used Magic wont load anymore, so one has to collect a lot of Magic Circuits. The Circuit was installed into Eliza, widely known as Cannibal Candy.
- Sonic (
音圧操作Sonikku): A Magic Circuit that grants an Automaton the ability to sense their surrounding, or the world itself by emitting sound through the return of the wavelength. Shoko described this Magic Circuit as excellent but difficult to utilize, as it requires an abundant amount of Mana to use; otherwise, the usage will be an arduous task and and the user will be burdened. So for this Circuit to live up its usefulness, living parts must be used. Sonic is currently installed in Rabbi and presumably in the rest of the Garm Series. It was installed into Yomi as well.
- Jet (
熱風操作Jetto): A Magic Circuit that grants an Automaton a type of Magic that can cut through something that a normal blade would not be able to cut, such as Yaya's durability. It also allows an Automaton to defy gravity, float, turn into, as well launching itself like a sword, giving an idea that the Circuit is based on Telekinesis. Furthermore, by channeling its exhaust heat, it can create a massive amount of thrust, and by combining them, it is possible to perform complex movements. If the exhaust heat is collected at one single focal point, it produces a very heated attack that can melt steel. Due to the exhaust it produces, one will think that the Magic Circuit is attributed to the utilization of the element of Wind. However, a Magic Circuit is not without a weakness; gunpowder can be used to counter the Magic Circuit's ability to melt steel because by producing an explosion, the gunpowder neutralizes the accumulation of heat. Cherubim and Lucifer are the known ones to have this Magic Circuit equipped into them.
- Yaegasumi (
八重霞Yaegasumi, lit. Eightfold Mist): This Magic Circuit entitles an Automaton to use a spell that enables them to enter in stealth mode by hiding all physical forms, muting all sounds and disguising all smells of its subjects. However, the ability only works with passive sensors; if one has active sensors equipped in them, they will be detected, deeming the spell ineffective. It is also said that the ability would be very demanding for a Puppeteer to handle. The Magic Circuit is currently equipped onto Komurasaki.
- Himokagami (
氷面鏡Himokagami, lit. Mirror of Ice): This Magic Circuit will grant the ability to: firstly, draw heat away from the surroundings of a target; secondly, make a huge icicle; and lastly, freeze water vapor in the air. Because of it, Irori has the simplest offensive power and regional control out of the three sisters. This Magic Circuit is installed into Irori.
- Fragarach (
完全統制振動Furagarahha): A Magic Circuit that was developed in Germany. It gives an Automaton the ability that controls atom vibration. With this Magic Circuit, a user is able to gain and exhibit superhuman durability, offensive power in terms of strength and increased movement by it. If someone throws the sacred sword to an enemy, it will fly to said enemy, attacks them before returning to the thrower by itself. Even with armor, no one can defend from its attacks. The current owner of this Magic Circuit is Sin.
- Ama-no-tajikara (
天手力Ama no Tajikara, lit. Great Physical Brute Strength): A Magic circuit that controls gravitation at will. It uses repulsive force and crushes objects. By making a gap in the gravitation between two points, it distorts space and changes configuration. It can also change the weights of targets, and the direction of weight. This Magic circuit was a compilation of Kyoshiro Karyusai's work, and is installed into Oborofuji series.
- Apparently, there are Heat, Momentum and Light Magic Circuits as well, but nothing is known about them so far.
- Despite accumulating 20 Magic Circuits in the manga, in the Facing "Cannibal Candy" IV episode, however, according to Felix, Eliza was capable of using forty-seven unique kinds of sorcery.
- Fragarach is named after the legendary sacred sword of the same name.
- Coincidentally, Ama-no-tajikara shares its name with Ame-no-tajikarao, a Japanese deity that is known for having an outstanding brute power, which also happens to be the scope of Oborofuji's power.