Magnetite

Magnetite (usually abbreviated MAG) is the living, mystical substance that demons' bodies are composed of in the physical world. Magnetite is a by-product of the physical world that is usually undetectable. Human bodies are the richest source of this mineral. In several series of games, magnetite is used to summon demons. The MAG cost to summon a demon is based on its level, as well as various other factors, such as the summoner's level and any difference in alignment between the demon and its summoner. In games where MAG is used, it is almost always rewarded after combat alongside experience, money, and whatever other spoils may have been earned.

Shin Megami Tensei Series
In the main Shin Megami Tensei series (I, II, and if...), MAG is used to summon demons. Demons that are currently party members consume MAG with every step the party takes. Some demons can be bribed with MAG during negotiation.

In Shin Megami Tensei III: Nocturne, MAG is not required, presumably due to the alteration of the universe and/or the protagonist's own demonic nature. On the other hand, given the extreme similarities between the two, Magnetite might be one and the same as Magatsuhi, which served a key purpose in the storyline.

In Shin Megami Tensei IMAGINE, Magnetite is used when summoning demons and performing certain skills (e.g. Megidolaon, Dynes and fusion skills). It can be purchased from other players, in addition to the normal method of earning it by defeating enemies in combat (drops).

Digital Devil Saga: Avatar Tuner Series
The term "Magnetite" only appears briefly in Digital Devil Saga: Avatar Tuner, in information provided by a terminal in the Karma Temple; here, it is stated to be the basis for Atma. Atma is the sustenance required by the demon-infected humans' Atma Avatars. They can only obtain this Atma by devouring other demons. This is also the mechanism by which party members learn new Mantras.

Devil Summoner Series
In the first two Devil Summoner games, MAG functions is the same way that it does in the earlier Shin Megami Tensei games. In the Raidou Kuzunoha sub-series, demons expend MAG only when summoned, either in the field or in combat.

Last Bible Series
In the Last Bible series, Magnetite is an item that is traded for special items at shrines (the equivalent of the Cathedral of Shadows).

Persona Series
Magnetite does not appear in the Persona series. Thus, Persona do not require Magnetite when equipped or summoned into battle. Instead, in Megami Ibunroku Persona and Persona 2, summoning a Persona uses some of the summoner's SP. The cost is based on the Persona, regardless of the skill being used. In later games, the cost is based on the skill being used.

Devil Survivor
In Devil Survivor, magnetite serves a completely different purpose. After a party attacks, they will be awarded a macca bonus based on how efficiently they fought. In addition, they will charge their magnetite meter, which is never displayed in any form. Once this meter reaches full, the player can assign an active skill to one of the demons in that party. Whether they chose to or not, the magnetite meter will drop to 0.