Immortality

Immortality (不死) is a state in which one will not die, be it through injury or age, when one normally would if they weren't.

Principles
Various forms of immortality have been theorized but they all involve those who are immortal not dying when they would if they were mortal. They would recover from injuries inflicted upon them which would prove fatal to a mortal, or simply not be affected by them at all. In cases where the immortality would prevent death of old age, the aging process would also likely cease at the point when the immortality was acquired.

Magical experts have theorized that since the mana that fuels magic is derived from a person's life-force, immortals, possessing unlimited life-force, would therefore have unlimited mana to fuel their magic (providing that the cycle of life-force which allows mana refinement hasn't been interfered with). Also, as immortals would have unlimited lifespans, they would be capable of acquiring spells which would be impossible for someone with a limited lifespan to do, such as Ars Magna.

Immortal Beings
Various creatures and beings are said to possess immortality. Immortality is one of the qualities often associated with Gods. Vampires are also said to be immortal, and they are feared and sought-after because of this. There are also certain mysterious beings such as Fräulein Kreutune who possess the trait.

Methods and Research
Many people have conducted research into methods to achieve (or in some cases, lose) immortality, with varying levels of success. Certain grimoires such as Baopuzi and the original Momotarou, are said to contain information on such methods. One method is the creation and ingestion of a special food or drink, such as is found in various mythologies. According to Niang-Niang, there are over one hundred legendary foods that can supposedly make one immortal on consumption. The elixir of life, traditionally associated with Alchemy, is one such example. Another is the Ambrosia, named in Greek Mythology. The one known person to have become immortal this way is Ladylee Tangleroad, who ceased to age after eating the Ambrosia and became unable to die. In this case, the Ambrosia twisted the cycle of her life-force, rendering her unable to refine mana and use Magic.

The immortal beings known as vampires possess the ability to turn those they bite into vampires, granting them the same immortality in the process.

Progressing down to the path of Magic to the extent of becoming a perfect Majin, or Magic God, appears to allow one to acquire a certain form of immortality. Othinus is believed to have lived for thousands of years and while in a perfect state, was able to withstand and recover from most injuries. Even weakened by Aleister's spell, the High Priest displayed similar invulnerability during his rampage through Academy City.

The magician Orlentz Trice believed that it might be possible to apply the defensive properties of original grimoires to a human body to enable eternal life, however his theory proved incorrect and his research ended in failure.

William Wynn Westcott, one of the Golden Dawn's founders, possessed a form of quasi-immortality (半不死). When describing it, Mina Mathers omitted the exact details but mentioned Westcott's speciality in forging documents, his access to corpses as a coroner and how parchment was used to converse with demons and transfer souls. It was later described as a state Westcott maintained by distorting the circulation of his life force using the attraction of a great being. Though Westcott never doubted what he considered the absolute strength of his quasi-immortality, Aleister Crowley killed him by firing an arrow bearing Imagine Breaker into his chest, tearing into the red mass in its center which contained Westcott's soul.

Aleister Crowley himself has been described as an immortal. Aside from having his lifespan extended by 1700 years due to his life support system, Aleister is also in a special state of existence in which he cannot be completely killed by conventional means, having assembled and sealed many potential versions of himself within his body.

Methods of killing immortals
Though immortals cannot normally be killed through ordinary means, there are some special means which are specifically meant to kill immortals. The Gemstone ability, Deep Blood, possessed by Himegami Aisa, is specifically meant to kill vampires, who are attracted by her blood and perish upon drinking it. The Fairy spell designed by Ollerus uses Christianity's tendency to reduce pagan gods into fairies to reduce a Majin in a similar way, severely damaging them in the process. In the case of Othinus, after the loss of Gungnir, the internal destruction proceeded to threaten her life. The spell that Aleister Crowley inserted into the High Priest, Nephthys and Niang-Niang is implied to be capable of bringing them to their deaths, and Kihara Noukan, using the Anti-Art Attachment connected to Aleister and delivering his willpower, was later able to kill the High Priest.

Ladylee Tangleroad made various attempts to find such a way to end her own life, without success - according to Index, even the world-threatening spell from the magic circle she had constructed in Endymion would not have killed her.