Vigor Mortis

Vigor Mortis (ヴィッガー・モーティス, Viggaa Mootisu, lit. "Vigour/Energy of Death") is a unique Caster Magic 'created' and utilized by Duncan Valentine.

Description
Vigor Mortis is, in essence, the ultimate last resort magic, at least for one who enjoys combat and spends most of their time fighting. It also appears to be unlearnable, or, rather, cannot exactly be taught to another, though the 'teacher' can place the student in the conditions required to learn it.

Essentially, when the user has been rendered almost dead, occasionally one's body will release a tremendous amount of energy in order to potentially allow the person to either defeat their opponent or escape. Vigor Mortis is the harnessing and controlling of this innate ability, now tying it directly to one's Magical energy. First, the user sets a specific 'pain/harm limit' on themselves, and it is this which controls when the magic activates. Once the user has reached this level of damage in a battle, their body will react instantly, letting loose with all the magical energy it can possibly release, causing an intense aura of magical power to explode into being around the user, usually doing great damage to the surrounding area. Within this state, the user's physical states are all maxed out, and they can utilize their magic to its highest extent, making them a veritable powerhouse while in this state.

However, this form/magic is not without its weaknesses, the most notable being the user themself. The magical energy they release while in this state is NOT bottomless, and depends heavily on how much magical power they are actually capable of releasing at that period of time. For that reason, at times the form will 'fail', either by refusing to activate if the user hasn't sustained enough damage, or simply tearing the user's body apart if their weakened body cannot handle the influx of magical power. Because of this, it is truly a last resort, as getting it wrong can lead to further injury, permanent disability, or even death.

Trivia

 * The magic was inspired by two things: First, the Hyper Mode function in Dragon Ball Z: Budokai 3, as well as the mechanic in several fighting games in which the player character gains a sudden burst of strength if they near death/knockout.