Forbidden Magic

Forbidden Magic (禁術魔法) is a classification for spells that draw power from the underworld.

Description
Forbidden Magic spells make use of negative mana (負の), a sinister mana from the underworld. These spells are more powerful than their normal counterparts and ignore the limits of normal magic. For example, Healing Magic cannot regenerate lost limbs, whereas Forbidden Magic can regenerate whole upper bodies. Curse Magic becomes more difficult, if not impossible, to break. Mages who use this magic are collected in the Arcane Stage of the Magic Stages system because their magic defies normal conventions.

Forbidden Magic can also permanently alter the souls and magic of users and victims. Licht's demonic transformation, the highest level of Forbidden Magic, destabilizes his soul so he is not fully awakened by the Reincarnation Magic. It also tainted his magic, as well as the magic of Lemiel who absorbed some of Licht's monstrous magic. Elves who succumb to despair and are influenced by Forbidden Magic become dark elves. Even if they recover mentally, their soul and magic remain altered. Humans who overuse Forbidden Magic risk developing Weg—a pair of black horns—and losing their humanity.

The magic stones have a connection to the underworld so they can be used to power and amplify Forbidden Magic spells.

Since devils are from the underworld, their spells could be naturally classified as Forbidden Magic.