Hellfire Incarnate

Fire Magic: Hellfire Incarnate (炎魔法『業火の化身』) is a Fire Magic spell.

Description
With an open grimoire, the user assimilates a large amount of Fire Magic into their body, which begins to glow and takes on a fire-like appearance, almost as if it has become Fire Magic itself. The user's strength is greatly enhanced, enabling them to punch aside the limbs of a giant demon and to knock it onto its back. The longer this spell stays active, the more potent it becomes.

However, a notable drawback is that if the spell remains active for too long, parts of the user's body will lose their shape and disappear because the body has become too much like mana and they will eventually die from overuse.