Deep Blood Arc

The Deep Blood Arc is a story arc that occurs in the Toaru Majutsu no Index storyline. It introduces Himegami Aisa and her ability "Deep Blood", which is central to the plans of Aureolus Izzard, a powerful alchemist and former partner of Index.

Aureolus Izzard and Index

 * Main article: Aureolus Izzard

Three years prior to the start of the Toaru Majutsu no Index storyline, Aureolus Izzard was an extremely talented alchemist of the Roman Catholic Church and was Index's partner. Though as to what extent their relationship was, it was evident that Aureolus Izzard cared for Index. Aureolus was unable to do anything when he discovered that Index needed her memories erased as so that she may live – as the holder of 103,000 grimoires.

Index's memories were erased, forgetting everything her memories of him. Aureolus defects from the Roman Catholic Church afterwards, determined to find a way to save Index from having her memories erased.

Himegami Aisa

 * Main article: Himegami Aisa

A few years before the start of the series, a remote village in Japan gets attacked by a vampire. The vampire turns every one residing in the village as a vampire, except for Himegami Aisa. The vampires act and look like regular humans, however, they were attracted to the blood of Aisa. Because of this, they were all killed by her ability, called Deep Blood; her ability attracts vampires to her, however, the vampires themselves are killed instantly because of her blood.

Aisa was horrified of her abilities, as many people she knew from the village died because of her. Because of this, she goes to Academy City, and enrolls in Kirigaoka Girls' Academy, believing that they can help her remove it, however, they were unable to.

Cult of Science

 * Main article: Cult of Science

Misawa Cram School, a preparatory school in Academy City, originally intended to spy on the workings of the Power Curriculum Program, goes roque from its original purpose and transforms into a cult of science. Aiming to replicate rare abilities and transfer them to other people, they abduct Himegami Aisa who has a rare ability, though they themselves were unaware of the details of her power.

However, Aureolus Izzard, who has now perfected his Ars Magna takes over the school, complicating Academy City's response to the situation. He then controls the minds of its inhabitants and puts a seal on the building making it his fortress. Himegami Aisa allies herself with Aureolus, after she realizes that Aureolus would need her powers to save someone. She goes outside the school from time-to-time as so she can attract vampires with her blood.

Their relationship, however, is misinterpreted as sort of an unlawful imprisonment of Aisa by Aureolus instead of a partnership between them. This situation, would later be used as the reason for the Church of England and Kamijou Touma's involvement.

Academy City's Response
Originally, Aleister Crowley simply planned on Academy City getting rid of the cult itself. However, since the arrival of Aureolus Izzard and his taking over of Misawa Cram School, the situation has become complicated, as the science side cannot simply intervene with the actions of the magic side, else it will disrupt the balance between the two sides.

Aleister invited the Roman Catholic Church and the Anglican Church to send in representatives to take down Aureolus, as such the Thirteen Knights and Stiyl Magnus was dispatched as the respectively for their churches. However Aleister did not tell either side there will be forces apart from themselves, leading the Thirteen Knights and Stiyl entering separately and resulting in the deaths of at least one of the Thirteen Knights.

Aleister suggests to Stiyl to use Kamijou Touma, the Imagine Breaker to aid him in the mission, citing that he is simply a level 0, a powerless person and that he will not be able to leak information from both sides. Stiyl highly doubts his reasoning as Touma's power defies understanding of both sides (or so he thinks) and no way is Touma a true "powerless" Level 0 like Aleister claims. It is even explained that it is almost like Aleister is deliberately giving Touma trials to overcome like the Saints of ancient times.

Prelude to Kamijou Touma's involvement
Touma tries to adjust to living the life of the old Kamijou Touma after the events of the Index arc. Although he struggles, he manages to fool Index in his ruse, and is fully aware that Index's feelings for him is that of the old Touma.

Despite this, Touma tries to continue on with his unusual life, living as someone else entirely for the sake of Index. His ruse is put to the test when Touma meets one of his friends from A Certain Highschool, Aogami Pierce who is curious regarding Index.

Touma had a little difficulty in trying to present himself to Aogami, however, he manages to fool him; Aogami however, is apparently too preoccupied with Index being with Touma. Unfortunately, Aogami's curiosity of Index got the best of him, annoying her. This forces Touma to calm her down, as he too provoked Index.

Meeting with Himegami Aisa
After much trouble, Touma decided on going to a rather crowded restaurant for Index, with Aogami (and Motoharu as well in the anime adaptation) tagging along. They are later forced to sit with a peculiar girl wearing a miko or shrine maiden outfit. Touma becomes wary of her, due to her queer apperance and becomes a bit hesitant to approach her.

He is however, reluctantly pulled into acknowledging the girl, after she states that she has spent her all of her money. Not knowing what to do, they decide to play rock-paper-scissors in order to find out who should talk to her. Naturally, Touma loses. Having to humor her, Touma discovers that the girl has used all her money up for hamburgers and would be unable to have enough money for the train fare back home.

After much frustration on her side, Touma eventually suggests for her to borrow money from someone. This suggestion however, piques her interest and looks upon Touma and the rest of the gang for such a money to borrow from, much to their surprise. Eventually, Index herself becomes irritated at her begging for money and off-handedly insults the girl being a shrine maiden. The girl however, corrects Index, and states that she is not a miko but a magician. This however, provokes Index even more, demanding Aisa to show some evidence regarding her claim. She then states that she should at least pretend to use an oriental-style magic since she looks like a shrine maiden.The girl however, disregards Index's arguments and simply states that she will be "that", which further infuriates Index.

Touma tries to calm the situation down, but is berated by Index for allowing the miko to get away with calling herself a magician, while she had to go to the trouble of proving the existence of magic to him. Their argument however, is suddenly interrupted by the suddent arrival of several suspicious-looking men, who has apparently arrived to take the girl back home. Despite Touma's suspicions, he cannot do anything about it. However, the girl relieves him of his worries and tells him that they are simply her cram school teachers. They and Aisa later leave much to their consternation.

Kamijou Touma's involvement
After leaving the restaurant and Aogami, Touma and Index comes upon an abandoned cat, and has apparently come to like it, and has already named it Sphynx. Touma strongly disapproves, however, the cat runs away, to which Index tries to go after it until she somehow notices a magical disturbance, leaving Touma alone.

It is revealed that Stiyl Magnus used his magic to make a distraction for Index just so he can meet with Kamijou Touma. Despite some trouble at first—to the point of having to himself from Stiyl attacking him—Touma manages to successfully Stiyl into thinking that he is still the same Kamijou Touma from before.

With Touma's full attention—to which made Stiyl somewhat suspicious of his perceptiveness—Stiyl explains that a girl has been in imprisoned in Misawa Cram School, and it up to him to free her. He notes on how the school has become a phony religious cult devoted to Science Worship. However, Stiyl states that whatever they are doing is no longer relevant as they have been taken over by an alchemist. Here, he states that the alchemist's goal was to abduct Deep Blood, a being who has the power to kill an being of legend, that even magicians don't even know exists, vampires. Stiyl states that the alchemist must be after the fact that since Deep Blood can kill vampires, by default vampires exist as well—a highly sought after being in the magic side.

Having enought of Stiyl's esoteric ramblings, Touma asks what he wants from him, to which he states that Touma is coming to aide him in rescuing Deep Blood. Before leaving him, howere, he states that the Church will take away Index from him if he does not comply.

Preparation for the assault
With Touma's recruitment, he is forced to take part in Stiyl's mission. Touma later returns to his dormitory to remind Index that he'll be late coming home. Also, here he finds Index and allows her to keep the cat she found a while ago, despite his protests.

Touma later discovers that Stiyl is planting runes of Innocentius outside of his dormitory room in order to protect Index from any one who may want to take her. During their short conversation, Touma discovers that Stiyl has an infatuation towards Index, which he tries to vehemently denies. Touma however, seeing his denial simply drops the subject.

Later, the two of them discuss their opponent, the alchemist known as Aurelous Izzard, and the captive Deep Blood known as Himegami Aisa. Here, he states that Aureolus Izzard has obtained the objective that the all alchemist pursuit and that is to model the world in his mind. More specifically, Aureolus knows and understands the laws of the world and can imagine the world in his mind, and in turn able to bring out that imagined world into the real world. In short, Aureolus Izzard had the ability to will that simulated world into reality.

Into the battlefield
Upon entering the building, Stiyl and Touma notice the eerie normalcy of the people inside the building. What's more, the people inside doesn't seem to notice their own queer presence in the building. Stiyl later explains to Touma that there is a seal that surrounds the building, separating people from a sort of innerside and outerside, as such they the students are unaware of the corpse inside the building, and when to objects from the outerside and innerside cross the same space, they will not interact and only slightly distort space. Stiyl also indicates that Touma's Imagine Breaker cannot destroy the spell unless he touches the core that is setup in the school for that boundary to take effect. As such they are trapped inside the school after going in.

At the same as this they discover the corpse of one of the Thirteen Knights of the Roman Catholic Church, here Stiyl explains that they may have been annihilated by Aureolus.

Although Touma is shocked to find a corpse, he tries to move on, as he is in a battlefield, with Stiyl's insistence, although he too is slightly unnerved by its presence, and sends the corpse off as a priest. Here, Stiyl notes that he has another reason to fight now.

First Assault
Touma and Stiyl are forced to use the stairs, seeing as they cannot interact with the objects on the other side of the barrier Aureolus made. While searching for the secret rooms, Touma checks to see if he can use his phone to interact with the other side of the coin. It works, as he is able to contact Index without a problem, and tries to make a suspicious excuse to Index. Stiyl however, chastises Touma for this as they are in the battlefield. He reminds Touma that the difference between him and Touma is that Touma succeeded in saving Index. Stiyl continues by saying that several other people have wanted to be in the position where Touma is right now—with Index. Touma takes this to heart, and is reminded by the fact that the Kamijou Touma who saved Index isn't him but the one who had his memories erased.

Gregorian Chant Replica
Still trying to search for the secret room, they later come upon a cafeteria filled with students, blissfully eating away. The students however, suddenly stopped doing what they were doing previously, and have apparently notices. Stiyl notes that the cafeteria had an alarm system that forces the students into "their" side. Here, the students began chanting a spell, which creates multitudes of magical spheres to attack them. With so many attacks to handle, both Stiyl and Touma decide to make a run for it.

As they are being chased by the spheres, Stiyl explains that Aureolus was probably able to wield the 2,000 students of Misawa Cram School as substitutes for the 3,333 monks that are normally used for the grand spell, a Gregorian Chant Replica. Stiyl notes that they need only to destroy the "core" that is controlling the students simultaneously in order for them to stop the spell. Stiyl and Touma however, are later separated because of the students' attack and Stiyl using Touma as decoy.

Touma later comes upon a girl who once again tries to use the Gregorian Chant Replica, however, she is hurt and is injured during her chant, due to her being an esper. However, due to Aureolus' control, she is unable to feel pain. Worried, Touma tries to stop her, but the girl faints afterwards, apparently overwhelmed by her own injuries. While preoccupied with the girl, Touma is nearly caught on by the spheres of magical energy of the Gregorian Chant. It surrounds him, but they later stopped moving and dissipates afterwards.

Immediately after, Touma meets with Himegami Aisa again.

Stiyl Magnus vs. Aureolus Dummy
Stiyl is shown to have detected the mana where the core of the synchronization of the Gregorian Chant replica is and has successfully destroyed it, which inadvertently saves Touma from the spell surrounding him. Immediately after, he is confronted by Aureolus Izzard himself who has apparently lured Stiyl intentionally with the Gregorian Chant Replica. After throwing a couple of insults towards Aureolus, he demonstrates his abilities to him, turning things into gold instantly, Limen Magna, and demands Stiyl to summon Innocentius so he can test his Limen Magna on it.

Stiyl however, refuses and belittles his Limen Magna, stating that using Innocentius would be overkill. Angered by Stiyl callous disregard of his ability, Aureolus attacks Stiyl with his Limen Magna, successfully hitting him several times. However, Stiyl is apparently unhurt by Aureolus, much to his surprise who continues attacking.Finally, Stiyl reveals to the troubled Aureolus that he is simply a dummy created by the real Aureolus. Stiyl quizzes the unbelieving dummy on why he studied Alchemy. Here, the dummy states that he wants to discover a way for a human to ascend higher without humans losing their humanity. However, Stiyl points out the flaw in his reasoning, asking the reason why he needed the vampires, who are beings that surpass humanity. With this, the dummy's world collapses entirely.

Suddenly, Stiyl appears behind him, apparently using a mirage all along during their conversation. Immediately after, Stiyl uses his fire sword, and hacks some of the dummy's limbs off. With this blow, the dummy goes wild, attacking things using his Limen Magna seemingly at random, completely destroying the floor and making it to molten gold. During all the commotion, the dummy took the time to run away from Stiyl.

Kamijou Touma vs. Aureolus Dummy
Meanwhile, Touma meets with Aisa again and discovers her motive on why she is helping Aureolus. However, shortly thereafter, Touma meets Aureolus and confronts him. Touma is however no match for him, and is simply erased of his memories and sent outside of the school with Stiyl.

Second Assault
Touma manages to dispel the effects of their memory erasure due to Ars Magna with his Imagine Breaker. They regroup and try to return to the school where they discover that remaining Thirteen Knights of the Roman Catholic Church has regrouped as well and fires the original Gregorian Chant on the Misawa Cram School. However, Aureolus counters the Gregorian Chant, and kills the 3,333 monks that are used for the spell in the Vatican, the building itself is repaired by the power of Aureolus' Ars Magna, thoroughly defeating the remaining knights.

Touma and Stiyl later discover that Index has wandered off into the school (after following Stiyl's mana trail, after worrying about Touma's whereabouts), and may have been abducted by Aureolus.

Final Battle
Stiyl and Touma confront Aureolus in his office, confirming their suspicion that Index truly was abducted by Aureolus. Here, Touma discovers Aureolus was a partner of Index from three years ago, and that he plans to save Index by turning her into a vampire, thus she will no longer have her memories erased annually. However, Stiyl tells Aureolus that Touma has already saved Index, and that Aureolus may not be aware of this because he was hiding from the Roman Catholic Church for so long.

Horrified that all his work was all for nothing, Aureolus kills Aisa with his Ars Magna after trying to save Stiyl and Touma - who was forced upon the floor - from being killed by Aureolus. Touma however, breaks away from the follor and negates Aisa's death with his Imagine Breaker and challenges Aureolus.

Touma negates every attack Aureolus uses on Touma using his Ars Magna, however, Aureolus sees one of the weaknesses of Imagine Breaker. He summons a gun into his hand that can fire a bullet fast enough that the human eye cannot perceive, thus Touma is unable to touch it with his Imagine Breaker. Touma is hit with several bullets but is not severely injured, but is told by Aurelous that he will not kill him easily. Stiyl however, mocks Aureolus' power saying on how he took an unnecessarily long way to get vampires if he could have created one himself using his Ars Magna.

Angered by this, Aureolus using his Ars Magna makes Stiyl float up and strips him of all of his skin. However, regardless of what occured to Stiyl, Touma is able to figure out the true nature of Aureolus' Ars Magna. Aureolus having enough of Touma and his Imagine Breaker, cuts off his whole right arm. Touma however survives this and begins to go after Aureolus which scares him. Knowing that Ars Magna is dependent on the user's state of mind to work properly, Touma uses Aureolus' fear of him against him, reverting Stiyl back and replacing Touma's cut off arm with a dragon, which he uses to defeat Aureolus.

Aftermath
Touma has his arm restored by Heaven Canceler, and had Aisa living with him and Index for a while. Aisa was also given a cross necklace by Necessarius as so she cannot attract vampires to her. Misawa Cram School is closed shortly after the incident. Aureolus loses his memories after being attacked by Touma, and has his appearance altered behind the back of the Roman Catholic Church.

The Roman Catholic Church is severely weakened after the arc, with 3,333 of their monks killed and several of their knights dead, the church is no longer able to perform one of their most powerful spells. This event cornered the Roman Catholic Church, which would later influence their actions against Academy City in later volumes.

Notable differences in the anime adaptation
Despite the anime adaptation's efforts to keep true with the novels, there are still several glaring liberties taken by the adaptation. One of the most egregious examples is the Deep Blood arc, taking a considerable away amount of content from the novels for the adaptation. Below is a list of notable examples: 

Trivia

 * This arc does not appear in the manga version, and Aisa is introduced instead during the Kazakiri Hyouka Arc where she enters A Certain Highschool as a transfer student and warns Touma about Kazakiri Hyouka. Stiyl Magnus is also later introduced much later during the Orsola Aquinas Rescue Arc.
 * According to Touma this arc occurs two weeks after he lost his memories which was July 28, this makes the date of this arc occur on 8 the 8th of August. In the Toaru Kagaku Railgun anime, Aisa is shown to have already acquired the cross necklace from Necessarius, which would imply that this arc has already passed. However, it is later revealed that the final date of the Poltergeist Arc is August 9, a day before the beginning of Sisters Arc in the Toaru Kagaku no Railgun manga timeline. We can infer that the there are timeline discrepancies in the Toaru Kagaku no Railgun anime adaptation.
 * The frog faced doctor points out in both the anime and the novels that in 10 days this has been Touma's second hospitalization, with that is possible to locate this arc in either the day 6 or 7 of August or perhaps even slightly earlier, that would mean that this arc does not conflicting with the others introduced in the Railgun anime.