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 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
Misawa Cram School, a preparatory school in Academy City becomes a cult of science. Aiming to replicate rare abilities and transfer them to other people, they abduct Himegami Aisa who has 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. Controlling the minds of its inhabitants and putting 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.

Academy City's Response
Originally, Aleister Crowley simply planned on Academy City getting rid of the cult. 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 Necessarius to send in forces to take down Aureolus, so the Thirteen Knights and Stiyl Magnus was dispatched. However Aleister did now 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. Stiyl cursed Aleister for playing this move, though says it is understandable since Science and Magic are different sides after all, and if the Magic side loses capable fighters it would be to Aleister's advantage.

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 that is the real reason 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. The narratives here in the novels says it's almost like Aleister is delibrately giving Touma trials to overcome like the saints of ancient times.

Preparation
Stiyl recruits Touma into his mission, with the threat that he will take away Index from him if he doesn't cooperate. Having no choice in the matter, he goes with Stiyl to Misawa Cram School after Stiyl applied runes to Touma's dormitory as so Innocentius can be summoned when Index is in danger.

There Touma discovers that the girl he met ealier in the Macronall Hamburger restaurant, is the same girl they are going to rescue.

First Assault
While inside the building, Stiyl and Touma 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. Stiyl also explains to Toum 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.

Later, Stiyl and Touma find out that Aureolus has setup a security system, forcing the students of the outerside and into the innerside. Here, they discover that the students have been synchronized to allow them to perform a replica of the Gregorian Chant, normally used by the Roman Catholic Church, and attack Stiyl and Touma. Stiyl and Touma are later separated because of the students attack.

Stiyl later discovers the seal that synchronizes the students that allow them to perform the Gregorian Chant Replica and destroys it, Stiyl later finds Aureolus and is forced to confront him, however, his mind is simply erased of the events that involved him and is sent outside of the school.

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 of his entire 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 crucifix 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.

Trivia

 * According to Touma this arc occurs two weeks after he lost his memories which was July 28, this makes the date of this arc approximately occur on 8 or 9. In the Toaru Kagaku Railgun anime, Aisa is shown to have already acquired the crucifix 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.
 * This arc does not appear in the manga version and Aisa is introduce instead in Kazakiri Hyouka Arc where she also enter A Certain High School as a transfer student and give warning about Kazakiri Hyouka to Touma.