Magic God

A Majin (魔神) or Magic God is a Magic side term used to refer to a being who has mastered magic, can bend all things in the world without exception through magic, something that transcends normal magic and steps into the realm of God, a Magic God. According to Silvia, the power of a Majin is even more troublesome than a great number of grimoires. With such an interesting prospect, many magic organizations, seek and fear the Majin.

Previously, it was thought that the title was held only by One-Eyed Othinus after supposedly stealing it from Ollerus, however, recent revelations debunked that notion.

Principles
Being a Majin is akin to being the purest, most perfect existence that encompasses every possibility. However, because of this, the Majin can also control both positive and negative possibilities. In essence, the Majin has an equal chance of attaining the possibility of succeeding as well as failing, making everything relating to the Majin's magic a 50/50 shot. This comes from the principle of the, which asks "if an omnipotent being can do anything, can it deny itself?"; i.e: could an omnipotent being create a rock it can't lift?

Moreover, this state in which a Majin steps into a realm of God, is compared to the Science Side's Level 6 by Kamijou Touma, a comparison that is not wholly farfetched as Level 6 requires a person to attain a body that can allow the person to understand the will of the heavens.

According to Othinus, in order to become a Majin, a magician has to master a path of magic until they reach the very end, so there can be multiple Majins who each have mastered a different path like Norse Mythology, Buddhism, Greek Mythology, etc

Failed Majin
There is also the possibility of being an impure Majin, meaning that the magician seeking to become a Majin stopped their growth right when they should have become one. Because of that, the impure Majin's overall powers are weaker, but there is no 50/50 restriction like a pure Majin.

Majins

 * Othinus is the one who later attained the legendary title of Majin from Ollerus, which Ollerus proclaims to have been stolen from him, making her one of the most powerful beings in the Toaru Majutsu no Index storyline. It is later revealed she had been a Majin a long time ago before she relinquished her powers. After finally unifying her infinite possibilities, she uses her powers to call upon Gungnir and destroy the world. . After the events of Volume 10 of New Testament, Othinus has lost practically the totality of her powers and now is about 15cm tall..
 * High Priest, a Japanese Buddhist mummy who became a Majin after his to become a  in a single generation was sabotaged.
 * Nephthys, an Egyptian mummy born of thousands of servants buried together with the pharaoh pooling together all the pieces of knowledge they possessed and they could find in the pyramid.
 * Niang-Niang, a Chinese ..
 * Other members from Gremlin.

Candidates

 * Index is the first character mentioned of being capable of being a Majin, with her having the 103,000 grimoires and is apparently immune to their poisonous nature. However, out of fear of having Index rebel and becoming a Majin, the Church has made provisions to disable her from using any spell by cutting off her mana. However, Index can still display some of the potential of a Majin's power when her John's Pen mode is activated, here, she shows tremendous power and can apparently use any spell from the grimoires, although her full potential have yet to be seen due to John's Pen Mode's restrictive thought process, and appears only in certain conditions, such as when John's Pen Mode's remote control is used.


 * Ollerus is the person that was supposed to have been closest in becoming a Majin that comes only once in 10,000 years but had let it slip through his fingers, when trying to save a kitten. Ollerus wields incredible power enough for him to be wanted by the entire magic side, and crushed all of his pursuers easily. Ollerus is an example of an impure Majin, yet he can still wield that power. Unlike Othinus, who is a perfect and pure Magic God, Ollerus is an impure Majin.

Discarded candidates

 * Sigyn is hinted to theoretically have the potential to reach the realm of being a Majin. If she was able to use her suggestion ability on herself, as her suggestions can make anyone succeed at 100% accuracy, she would be perfect. However, she cannot do it, since her nature is to give what she has to others to make up for what they lack. If she tried to be perfect herself, she wouldn't be able to make up for what she lacks.

Trivia

 * All Majins and valid Majin candidates shown in the story have green eyes. The only exception to this is Nephthys, who as is mentioned in the novels and seen in the illustrations, uses her powers to change eye colors at a whim. However, she does have green teardrop tattoos below her eyelids.