Sacred Gear

Sacred Gears (神器（セイクリッド・ギア） Seikuriddo Gia), also known as God's Artifacts, are powerful abilities bestowed upon humans by the original God from the Bible. They are one of the main points of interest in the High School DxD series.

Summary
It is explained that the original God created the Sacred Gears as part of His system to enact miracles on Earth. It is also mentioned that certain individuals with Sacred Gears have become very powerful and influential, but no example has been given. There can be more than one Sacred Gear of the same type such as Blade Blacksmith. There also appears to be common Sacred Gears, like the Twice Critical and rare Sacred Gears like Twilight Healing. The only exceptions being the Longinus, which are all unique. If the Sacred Gear is removed from their owner, the possessor will die.

Only humans or human hybrids are born with Sacred Gears. Angels and Devils can obtain them by resurrecting a human or human hybrid as one of their own. The Fallen Angels have devised a ceremony to extract and possess a Sacred Gear from the original user.

It is possible for an individual to own more than one Sacred Gear. As to why Angels and Devils are never born with them, yet keep them when transformed from humans, is another mystery. Presumably because God never foresaw the new systems of reborn Devils and Angels which arose after His death.

Abilities
Sacred Gears can have a variety of effects, such as Asia's Twilight Healing granting near-instantaneous healing and Siegfried's Twice Critical plus double the power of the user. Yuuto's Sword Birth, for example, can create swords imbued with different attributes, examples include creating a sword that absorbs holy energy, a sword that devours flame, and a sword that absorbs wind.

Sacred Gears also adapt and evolve based on their wielder's thoughts, needs, and feelings, gaining abilities or qualities as time passes. This is stated to be the source of Balance Breaker, and suggested to be the source of the transformation into a Sub-Species.

Balance Breaker
Sacred Gears have an ultimate state of activation called the Balance Breaker, which is the most powerful manifestation of the Sacred Gear. Once reached, the Balance Breaker can be entered again far more easily than the first time. An incomplete Balance Breaker can be reached by self-sacrifice on the part of the user, such as when Issei sacrificed his left arm, allowing it to be transformed into a Dragon arm. However, this sacrifice method does not count as a true activation of Balance Breaker. Reaching the Balance Breaker can be triggered by the feelings of its possessor. Balance Breaker is said to be have been never intended, and that it was a flaw in the system created by God.

Juggernaut Drive
Juggernaut Drive is a special ability exclusive to the Longinus Boosted Gear and Divine Dividing, and are considered to be forbidden moves. The Juggernaut Drive temporarily removes the seal placed on the Heavenly Dragons, but the users will lose their sanity while their lives are devoured by the power.

For other Sacred Gears that have creatures sealed inside (such as the Regulus Nemea), their Juggernaut Drive goes under a different name called "Breakdown the Beast" (覇獣（ブレイクダウン・ザ・ビースト） Bureikudaun za Bīsuto).

The True Longinus has a similar power as well, which unleashes the 'Dying Will' of the God of the Bible called Truth Idea.

Artificial Sacred Gears
It is possible for artificial Sacred Gears to be created. Azazel, the leader of the Fallen Angels and Grigori, has conducted countless research on Sacred Gears. He eventually manages to create his own Sacred Gear, the Down Fall Dragon Spear, using the jewel of Fafnir, one of the five Dragon Kings. Artificial Sacred Gears, however, are inferior to the real ones.

Sub-Species
Sub-Species are Sacred Gears that have taken on a unique characteristic because of the thoughts and feelings of the wielder. They are more powerful than the normal form of the same Sacred Gear, and will sometimes gain a new ability as well. Sometimes, both the Sacred Gear in its normal state, as well as the Balance Breaker will be a Sub-Species, Seigfried's for example, but other times, only the Balance Breaker will be a Sub-Species, such as Yuuto's Blade Blacksmith. Even Longinus can gain a Sub-Species Balance Breaker like Cao Cao's True Longinus' Polar Night Longinus Chakra Valdine and Sairaorg's Regulus Nemea's Regulus Rey Leather Rex.

Issei has been told by Azazel that he should develop a Sub-Species for Boosted Gear. This suggests that Sub-Species can be made both intentionaly and unintentionaly.

Longinus
Aside from the normal Sacred Gears, there are the Longinus. Longinus are unique top-tier Sacred Gears, each having multiple abilities compared to a normal Sacred Gear which only has one, and the power to slay Gods. This class of Sacred Gears is also known as Tools that Destroy God. There are 13 Longinus in total. However, Azazel has stated that there is the potential for more Sacred Gear to eventually evolve into Longinus, under the right conditions. Also, that Longinus and Balance Breakers were never intended, and are glitches in the system. The Longinus class of Sacred Gears are named for the first Sacred Gear to achieve this level of power, the True Longinus.

Dragon-Type Sacred Gears
Some Sacred Gears gain a Dragon quality to them. This can be because of having a Dragon sealed within them, as Boosted Gear or Divine Dividing do, or by assuming a Dragon form, such as Seigfried's Sub-Species Twice Critical. Those who wield these Dragon-Type Sacred Gears are normally more powerful but become partially vulnerable to Dragon Slayer weapons, such as Gram or Ascalon, or the curse of Samael, the Ultimate Dragon Slayer.

Holy-Type Sacred Gears
Some Sacred Gears have a special property to them that allows them to produce Holy energy, much like Holy Swords, or blessed or sacred items do. This makes them dangerous weapons against Devils. Examples include Blade Blacksmith, and the True Longinus.

Demonic Sacred Gears
Demonic Sacred Gears are those with the ability to use Demonic powers of some kind. The only known example at present is Yuuto Kiba's Sword Birth. It is unknown if a Demonic Sacred Gear causes the user to be vulnerable to Holy items, like Dragon-Type Sacred Gears render their users vulnerable to Dragon Slayer items, as the only known user is a Devil, and thus already vulnerable to them.

Chaos Break and Chaos Drive
A special doping for Sacred Gear made from the blood of the original Satans, created by the Hero Faction with the help of the Old Satan Faction. The doping, called Chaos Break, allows the Sacred Gear to undergo Chaos Drive, in which the Sacred Gear (and Holy/Demon Swords) fuses with the user while transforming the user into a monstrous form. The names are taken from the Balance Breaker and Juggernaut Drive. In Volume 12, it is stated that after the first use, each subsequent Chaos Break will reduce the user's lifespan. Chaos Drive also renders one immune to Phoenix Tears for unknown reasons.

Trivia

 * Juggernaut Drive was originally stated to be a special ability which was exclusive for Sacred Gears with powerful creatures inside. Though in Volume 11, it was changed to a special ability exclusive for Sacred Gears with Dragons residing in them (Boosted Gear and Divine Dividing). For other Sacred Gears with other creatures residing in them, a new name, called Breakdown the Beast, was given.
 * In Volume 4, Azazel claims that the Sacred Gear system is incomplete due to both God and the Four Satans died while leaving "bugs" such as the Balance Breaker and Longinus behind.