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 original God, 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 is 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.

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 Issei could only maintain it for short periods of time, undergoing intense training helped him improve this time. When first introduced, Vali could maintain his Balance Breaker for a month. The forms of each Balance Breaker differ according to the user.

Sub-species Balance Breakers
Balance Breakers can also have a sub-species. The sub-species Balance Breaker creates a diferrent form than the actual ones and are usually more powerful than them. Sub-species 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 sub-species Balance Breaker is currently unknown but it seems to involve the user's personality.

I love your oppai

Trivia

 * Currently, the number of sub-species Balance Breakers outnumber the actual ones.
 * In the short story "Rias in Wonderland", 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, it seems, 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.