The Balance Breaker, also known as the Forbidden Move, is the evolved form of Sacred Gears in which their full powers can be released.
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 Sub-Species, could be placed under the Crest Side 昇華面, Kuresuto Saido. The Crest side is classified under strengthening, evolving to a point of transformation. By pursuing the arrogance of themselves and the Sacred Gear, one can further evolve the Balance Breaker creating a fusion state with their Sacred Gear to produce the Abyss Side 深淵面, Abisu Saido. The third type, Ex Side 慮外面, Ikusu Saido consist of the Balance Breaker that couldn’t be categorized under the previous two categories due to their sudden variation, and is considered as exceptions among exceptions.
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. Marsilio 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.
Crest Side Balance Breakers
The regular state of Balance Breaker through strengthening. It consist of a Sacred Gear's original Balance Breaker. It is the most common state of Balance Breaker. The Sub-Species Balance Breaker also consist of this category. The Sub-Species Balance Breaker creates a different form than the actual ones. 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.
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]. Vali Lucifer's Empireo Juggernaut Overdrive is also considered an Abyss Balance Breaker due to Vali attaining it by sealing the past wielders of Divine Dividing.
Ex Side Balance Breakers
This class of Balance Breaker belongs to a category that can't be classified under any of the known category due to being an irregular, either due to sudden change or it being a combination of the known categories. Issei's Hyoudou's Illegal Move Triaina and Cardinal Crimson Promotion 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.
→ Polar Night Longinus Chakravartin : Cao Cao's Sub-Species Balance Breaker. It creates a ring of light and seven orbs around him. Each orb providing him with a special ability, and the True Longinus is still a spear which he can use.
→ Flagello di Colori del Arcobaleno, Speranza di Briscola: Dulio Gesualdo's Sub-Species Balance Breaker of Zenith Tempest. In this form Dulio possesses 12 golden wings, and four halos above his head. Dulio creates countless bubbles, trapping his opponents within them. Dulio then controles the climate within these bubbles to attack those trapped within, creating Intensely swirling flames, violent hurricanes, freezing cold air which freezes everything, and vicious torrents of lightning.
→ Bandersnatch and Jabberwocky : Leonardo's Sub-Species Balance Breaker. It creates thirteen gigantic monsters (one Jabberwocky and twelve Bandersnatches). Both type of monsters have the ability to create smaller copies of themselves, while the Jabberwocky has enhanced regenerative abilities allowing it to heal rapidly from any external injury and reattach severed limbs.
→ Regulus Rey Leather Rex : Regulus Nemeas's Sub-Species Balance Breaker. It covers the user in a Golden Lion Armor that highly augments the wielder's power. It also provides him protection from projectile attacks.
→ Night Celestial Slash Dogs: Tobio Ikuse's original Balance Breaker. Tobio transforms with darkness into a humanoid black Hound with six tails and Jin transforms into a big Black Hound, in this form not only does Tobio gain enhanced strength he can create an innumerable amount of blades, capable of tinging the sky and earth in darkness.
→ Perfectus Tenebrae Lykaon Fortis Densus Laelaps: Tobio Ikuse's Abyss Side Balance Breaker. He achieved this form after training and becoming accustomed to using his Sacred Gear. By reciting a curse-like chant, Tobio’s entire body becomes covered in darkness, and he takes the form of a humanoid beast clad in darkness and he gains a long and immensely sharp scythe and can also create jet-black gigantic blades from the ground.
→ Incinerate Antiphona Calvario : Walburga's Sub-Species Balance Breaker. Its characteristics and capabilities may vary depending on the crucified soul Walburga imbues it with. The forms are able to move independently from her and acts like an independent-avatar type.
→ Lovely Heavenly Angels: Lint Sellzen's Sub-Species Balance Breaker. It creates 3 huge angels (2 males and 1 female representing Siegfried, Freed, and Lint) covered in purple flames. The angels can combine with each other to become one large angel.
→ Twilight Saint Affection: Asia Argento's Sub-Species Balance Breaker. It takes the form of a Golden Dragon Armor with red jewels that covers her body and has the ability to cover a field with tremendous healing property that can negate any damage from virtually any sources.
→ Blade Knight Mass : Blade Blacksmith's Balance Breaker. It creates multiple armored knights equipped with a holy sword.
→ Stake Victim Dragon : Jeanne's Sub-Species Balance Breaker. It creates an artificial dragon made entirely of holy swords.
→ Glory Drag Trooper : Yuuto Kiba's Sub-Species Balance Breaker. Similar to Blade Knight Mass, it creates armored knights equipped with a holy sword. It differs from the Blade Knight Mass as Glory Drag Trooper's knight's armor are dragon-themed and they reflect Yuuto's speed and technique. Due to his inexperience, the Dragon Knights can only reflect his speed and not his techniques.
→ Night Reflection Death Cross : Connla Sub-Species Balance Breaker. It creates an armor made out of shadows. It grants the user to become like a shadow, gaining its intangible properties and allowing physical attacks to pass right through him, although the user is still susceptible to most forms of energy (heat/cold).
→ Malebolge Vritra Promotion : Genshirou Saji's combined Sacred Gear's Ex Side Balance Breaker. It covers the Saji in a black dragon armor with tentacle-like things growing from it. In this state, Saji and Vritra's consciousness is fused into one and Vritra's powers can be displayed more than when using Vritra Promotion.
→ Down Fall Dragon Another Armor : Down Fall Dragon Spear's burst-state Balance Breaker. It covers the user in a golden armor much like Issei and Vali's Scale Mails. It mainly strengthens the user's overall power using Fafnir's power.
→ Fourteen Holy Salvation: Balance Break that allows user to increased the area of effect as well as a wider range of symptoms that could be cured. If the target was a believer, even a serious illness could be completely cured.
→ Sturdy Saint Withstand: In this state, the user's defense is enhanced to the point that they can survive an attack equivalent to Xenovia's Cross Crisis after the latter had been greatly strengthened by Crimson Destruction Dragonar.
• Dreamlike Curse
→ Paraselene Utopia: It has the ability to send a targeted individual into a barrier space of his creation (a seemingly parallel world). One of the rarer Balance Breaker abilities, as it doesn't seem to have direct attack powers, it is often used in conjunction with illusions to create a sense of despair for both the targeted victim (as well as crumbling their mentality) and those from the other world who they share a connection with.
Issei's Balance Breaker - Boosted Gear Scale Mail
Kiba's Ex Side Balance Breaker - Holy-Demonic Sword, Sword of Betrayer
Vali's Balance Breaker - Divine Dividing Scale Mail
Azazel's Balance Breaker - Down Fall Dragon Another Armor
Connla's Sub-Species Balance Breaker - Night Reflection Death Cross