Necromancy

Necromancy (死霊術 (ネクロマンシー)) is a type of magic that is manipulates the dead, and is grouped alongside Hopping Zombie Spell, Hand of Glory, and Vetala Sorcery.

A user of Necromancy is called a Necromancer.

Principles
With the use of Necromancy, rituals can be carried out on the bodies of the dead to allow them to "pass through the gates of Heaven". These rituals involves putting back the wandering "residual memory" of the corpse and closing their "spiritual circuit", in order to put them to rest. To do this, the necromancer makes use of a magic array to reactivate the corpse and sends a false soul (偽りの魄) into the body in order to carry out the process.

If the "residual thoughts" are not controlled, they can take a dangerous form sometimes called ghosts. Residual memory can gather outside of the corpse, appearing in the form of a floating brain. If someone touches it, the memories of the deceased will flow into their mind.

The false soul used to possess the corpse, for the purpose of closing the spiritual circuit, is capable of operating the body it is possessing, such as when Estelle Rosenthal's Huotou (禍斗) activated after she was interrupted before she could close Hitokawa Hasami's spiritual circuit and acted to defend it's master.

Background
The word "necromancy" is adapted from Late Latin necromantia, itself borrowed from post-Classical Greek νεκρομαντεία (nekromanteía), a compound of Ancient Greek νεκρός (nekrós), "dead body", and μαντεία (manteía), "prophecy or divination." As the name suggests, necromancy is the magic of manipulation of the dead for communication for the purpose of divination.

Kazakiri Hyouka Arc
It is referenced after Index sees Kazakiri Hyouka's leg regenerate completely after kicking Sherry Cromwell's golem down, and saving her.

Toaru Kagaku no Accelerator
Accelerator encounters a necromancer, Estelle Rosenthal, during the events concerning the rogue Anti-Skill group DA, and Estelle uses necromancy on the corpse of Hitokawa Hasami.

List of known Necromancers

 * Estelle Rosenthal