Balance Breaker

The Balance Breaker, also known as the Forbidden Move, is the evolved form of Sacred Gears in which their full powers can be released.

Summary
Azazel claims that Balance Breaker is a "bug" in the system created by the God from the Bible, which can upset the balance of the world and thus given the name Balance Breaker.

The Balance Breaker is initially considered a forbidden power by the Three Factions. It is known as the ultimate state of activation for a Sacred Gear.

Balance Breaker is often triggered by high emotions combined with a change in the heart or spirit of the wielder. It can also be activated when the user is faced with a disadvantageous situation, such as the time when Issei had to go up against both Bikou and Kuroka in Volume 5.

According to Azazel, the possibilities of the Balance Breaker can be divided into three categories. The majority of Balance Breakers, including the subspecies, could be classified as strengthening, evolving to a point of transformation. By pursuing the arrogance of themselves and the Sacred Gear, one would fuse together with their Sacred Gear to produce the Abyss Side. The third type consist of the Balance Breaker that couldn’t be categorized by the previous two categories due to their sudden variation, and is considered as exceptions among exceptions.

Abilities
The Balance Breaker is the most powerful manifestation of all Sacred Gears in which the possessor's feelings trigger this evolution. Once reached, the Balance Breaker can be entered again far more easily than the first time. With proper training, it can be maintained for longer periods of time, as with the case of Vali. While initially both Yuuto and Issei could only maintain it for short periods of time, undergoing intense training helped them improve this time. When first introduced, Vali could maintain his Balance Breaker for a month.

As revealed in the short story "Rias in Wonderland", the Balance Breaker can also be lost. The remaining remnant of the Hero Faction was discovered to have lost his Balance Breaker ability after he was sent to the Underworld. The reason is quite possibly due to his feeling of "satisfaction" since he was able to retaliate against the Gremory group. This means that similar to how a change in one's feelings can "unlock" the Balance Breaker ability, another change in the opposite manner could have the reverse effect.

Subspecies Balance Breakers
Balance Breakers can also have a subspecies. The subspecies Balance Breaker creates a different form than the actual ones and are usually more powerful than them. Subspecies Balance Breakers have a tendency to take the form of the user's desired ability in addition to the original. The method to attaining a subspecies Balance Breaker is currently unknown but it seems to involve the user's personality.

Abyss Side Balance Breakers
By desiring to further improve oneself after attaining the Balance Breaker, it's possible for the Balance Breaker to undergo further change. Tobio Ikuse awakened the [Perfectus Tenebrae Lykaon et Fortis Densus Laelaps] by further polishing the [Night Celestial Slash Dogs].

Irregular Balance Breakers
This class of Balance Breaker belongs to a category that can't be classified under any of the known category, either due to sudden change or it being a combination of the known categories. Issei's Hyoudou's Triana and Cardinal Crimson which is made possible through the Evil Pieces, Genshirou Saji's Balance Breaker is formed through the combination of his 4 Sacred Gears into one are among the known examples of this category.

Trivia

 * Currently, the number of subspecies Balance Breakers outnumber the actual ones.