Necromancer Arc

The Necromancer Arc (死霊術師編) is the first story arc of Toaru Kagaku no Accelerator. It deals with Disciplinary Action or DA, a rogue extremist group of Anti-Skill, and the parties manipulating them who make use of necromantic techniques.

Background
On August 31st, following the suspension of the Level 6 Shift Project after his defeat by Kamijou Touma, Accelerator was shot in the head by Amai Ao while saving Last Order. Although he was saved by Heaven Canceller, the brain damage he sustained left him reliant on the Sisters he had once killed, via a choker connecting him to the Misaka Network, in order to process language and perform calculations. During the first weeks of September, Accelerator remained an inpatient at Heaven Canceller's Hospital.

Rosenthal Family
The Rosenthal Family are a line of necromancers who use a style of necromancy born from a mix of Kabbalah and Taoism, involving artificial souls stored in charms which are inserted into corpses to animate them, and have sought the creation of the perfect golem through the development of these techniques.

Over 400 years before the present, having tested various methods of creating golems, the Rosenthal's ancestor Obed, devised a method of inserting an artificial soul into a corpse, allowing them to use a human brain and greatly increasing their intelligence, to create intelligent golems as opposed to rock dolls only capable of carrying out simple commands. The of his homeland wouldn't permit this and drove Obed away from his home as a heretic.

Ending up in the East, Obed and his successors would continue to develop their techniques, with the 4th generation head Isaac, incorporating the Hopping Zombie Spell and the 5th generation Nathan creating four artificial souls with greatly increased abilities; Taowu, Taotie, Hundun and Qiongqi, sometimes referred to as the Evil Spirits of the Numbers. Before Isaac died from disease, Nathan transferred his soul into the Taowu, imprinting it on its artificial soul. Knowledge of this transfer was lost at some point. The Rosenthal style would continue to develop over the subsequent 19 generations but their ultimate goal of creating the perfect golem, one with a perfect soul and body - a god, continued to elude them.

Esther and the Hishigatas
For five years, neuroscientist Hishigata Mikihiko had been involved in research which part of the body is the source of an esper's ability. The team he was in carried out dissection of bodies and the removal of parts from living specimens to see if they still had abilities afterwards. He eventually came to the conclusion that it had something to do with the soul, but also came up with the idea of reinforcing and expanding the body in order to enhance the ability. At some point, Esther Rosenthal, 23th head of the Rosenthal Family, joined his research team as a supervisor. According to Esther later, she was apparently tricked into doing so. Regardless, she shared her knowledge of necromancy without fully grasping the consequences, enabling them to make considerable progress and eventually leading to the development of the Coffins. During this time, Esther also became friends with his younger sister Hishigata Hirumi, a normal student attending Seiin High School.

However, Hirumi ended up dying under unknown circumstances and Esther tried to revive her using Taowu. When the Taowu, containing the will of Isaac, was activated (for the 17th time), it was contacted by a self-proclaimed demon, who conveyed a way to achieve the Rosenthal Family's desire, using a method derived from how the human soul is said to slowly evolve and reach Keter through the process of experiencing 10000 deaths. By converting the memories of death into a special formula and adding it to the sephiroth constructing the artificial soul, engraving the proof of death into the soul 10000 times, their soul would also reach Keter. The memories of death are normally erased from the physical world at death, however the memories of 10031 deaths are present in Academy City, stored within the Misaka Network.

The Taowu, influenced by the will of Isaac, moved to carry out this method and accomplish the Rosenthals' goal, posing as Hirumi to manipulate her brother and presenting the plan to him in the guise of making Hirumi into a Level 6. Esther soon realized that she had lost control of the Taowu and that whatever will was guiding it planned to interfere with the processes of life, death and reincarnation, potentially altering the world at a fundamental level. As a result of losing her friend twice, Esther came to understand the weight of life and death, and realizing that they should not toy with death, she abandoned the desire of the Rosenthal Family. Following this change of heart, she ended up being held captive while Hishigata continued to develop the Coffins using the techniques she'd taught him. She eventually escaped and sought out Last Order, having learned that she would be targeted to obtain the memories necessary for the plan from the Misaka Network.

DA
Disciplinary Action, commonly abbreviated to DA, was an armed organization which began as a group of Anti-Skill officers who received disciplinary action for going overboard in their law enforcement. A cooperative organization on paper, it was actually a secret society of zealots and extremists, willing to use any means necessary to carry out what they call "justice". Comprised largely of disciplined and disgraced members of Anti-Skill, the group was supported by numerous sympathizers, including current and former members of Anti-Skill, certain influential figures and rejected defense contractors, supplying them with information, funding and equipment, with technology surpassing that of Anti-Skill. One of their backers was board member Nakimoto Rizou, who believed the group could be useful for him and kept that information on the group withheld from Anti-Skill.

Hishigata Mikihiko joined DA at some point within the past few years and provided them with the Coffin technology which he had developed from Esther Rosenthal's Necromancy techniques, though he merely planned to use the group for his own ends.

Three days before the incident at Heaven Canceller's Hospital,  several DA members came across Hitokawa Hasami after a failed suicide attempt and murdered her in the ambulance, drowning her with river water through her oxygen mask, in order to make use of her ability in the Prototype Coffin.