Devil

The Devils (悪魔 Akuma) are one of the Three Factions in High School DxD, alongside the Angels and Fallen Angels. There are two major types of Devils in the story: The Pure-Blooded Devils (純血悪魔 Junketsu Akuma), and Reincarnated Devils (転生悪魔 Tensei Akuma).

Summary
Devils are beings that originated from the Underworld consisting of the 72 Pillars of High-Class Devil noble families, the Extra Demons and the countless armies under them. The total life span of a Devil is approximately 10,000 years. The Devils were originally led by the Four Great Satans prior to their death in the Great War. Following the end of the great war, the descendants of the original Satans apart from Rizevim, wanted to continue the Great War but were faced with opposition by other Devils, most prominently the Bael clan, who assumed authority after the death of their leaders. This started a civil war between the Devils with the descendants of the original Satans losing the war and a new government was formed among the Devils. The new government then appoints four new Satans through strength and accomplishment in the civil war.

After the formation of the new Devil government, Ajuka Beelzebub developed the Evil Pieces to help replenish the decreased armies of the Devils, eventually creating the Rating Games.

Appearance
Most Pure-blood Devils look exactly like humans, albeit with at least one pair of black bat-like wings that can be hidden at will. However, those from the Phenex family do not have the usual bat wings, instead having eagle-like wings of fire. Some pure devils can also have pointed elf like ears. It is still unknown if Devils can have multiple wings like the Angels and Fallen Angels, though Vali, who is a Half-Devil/Half-Human, has shown Issei "multiple" pairs of bat wings in Volume 4 (with four pairs in the anime) and Rizevim Livan Lucifer has shown to have twelve bat wings in Volume 18. Reincarnated Devils can come in a variety of shapes and sizes, such as the case with Tannin, a former Dragon King, Koneko Toujou, a Nekomata, and Enku (a Qilin).

Devils don't share the same eye and hair colour like Angels and Fallen Angels do except from their parents. Female Devils tend to utilize magic in order to appear much younger than their physical age would suggest, which explains why Venelana Gremory looks as if she were the same age as her youngest child, Rias. There are monster Devils or Devils with the characteristics of a creature such as the members of the extinct House of Vepar being Devil-Merpeople, or the members of the House of Forneus having the head of a sea monster like an anglerfish.

Abilities
All Devils have increased physical abilities, such as superhuman strength, endurance, with enhanced senses such as an increased hearing range and eyesight. In addition, as they are creatures of the night, Devils have the special ability to see clearly in the dark. Devils also have the ability of flight through their bat-like wings, their wings are also capable of being retracted into their backs as if they've disappeared completely. All Devil possess a passive ability, called "Language", where people listening to them will hear it in the language they are most familiar with; and vice versa. According to Rias in Volume 4 of the light novels, Devils have an immunity to being sunburned.

High-Class Devils are able to gain peerages, rule over other Devils depending on their family's rank and powers, and can have up to 15 "servants", which are represented by each piece in a chess game (Rook, Knight, Pawn, etc.). The High-Class Devil takes the position of "King". Lower-Class Devils are often looked down upon, and usually serve a Devil of higher authority. High-Class Devils also have the ability to reincarnate Humans and other races into Devils through the Evil Pieces system, as seen with what Rias did with Issei and Asia Argento.

Just one of the Four Great Satans could destroy the nation of Japan several times over with their power. Whether this is true for all Ultimate-Class Devils or not is unknown, but it does give an indication of their power.

Members of High-Class clans appear to inherit special innate magical abilities genetically unique to that clan, though occasionally descendants of those who marry out of the clan will also inherit these abilities. Like those of the Phenex's Clan who have the power of immortality, the Bael's and Gremory Clan who has the Power of Destruction, and the Abaddon's Power of Hole. The origins of these powers are largely unknown. The pure-blooded Devils who don't inherit such powers are looked down upon, even by members of their own family.

Devils also have the ability to cast magic spells, not limited to those in which their family specializes. Examples being the ability to bind and later summon a Familiar, as well as Issei's Dress Break and Pailingual spells. The most common usage would likely be the magic circle used by Devils for travel, and especially to make contracts.

As revealed in Volume 14, to use demonic powers, a Devil must first acquire the power of imagination and the power to create, as well as have a good sense. Devils with higher demonic power can develop resilience to holy objects and light weapons.

Weaknesses
A common weakness for all Devils is that they cannot go near/get involved with anything that has to do with the Biblical God, as they will suffer nasty side effects (headaches, immense pain, etc.), as seen when Issei couldn't go near the Church when he was talking to Asia, or when he tried to pray but only ended up collapsing from the pain soon after. They are vulnerable to holy objects such as crucifixes, holy water, and the Bible, and can get killed easily by Holy Swords and Sacred Gears with divine properties, such as Blade Blacksmith and the True Longinus (Asia's Twilight Healing, however, is a notable exception). These effects are caused by the system put in place by the Biblical God, which is now being kept up by Gabriel, and only she has the power to change the system.

They are also susceptible to light which significantly drains them of their energy once hit (in Volume 1, Issei comments that sunlight also appears to drain him of his energy, reducing him to normal human levels, at least at that time). Also, intense light, such as the light-based weapons used by Angels and Fallen Angels and the light swords used by exorcists, can severely wound, if not kill, a Devil. Devils with higher demonic power, however, can develop a certain degree of resistance to light.

Devils can also be infected with a cold that only Devils are susceptible to as seen when Issei and Saji came down with it, which made them unable to move. The Devil's face turns red and they gain a high temperature along with blurry vision.

Ranking
The Devils use a hierarchical system that is split into four different ranks. The ranking here refers to the hereditary title given to Devils base on their classes and clans and do not reflect on strength. They are:

There are three different ways to raise one's ranking: Once a Low-Class Devil has earned the right to raise their ranking, they must first be tested before actually gaining the position. Not everyone succeeds in these tests, but they can try as many times as they choose, as the permission does not normally expire. The Middle-Class promotion test has three parts that are modeled after the tests in the Human world since a majority of the Devils who taking the promotion tests are reincarnated Devils (former humans). The first part is a written report that addresses the theme of aim and greed. The second part is written test involving the history and government of the Underworld. The third is a practical where an individual faces off against another participant from the test in a one-on-one fight. During this battle, Pawns are given special permission that allows them to promote as if they were in enemy territory. High-Class Devils and above are allowed to have their own "peerage" which consists of members who represent each piece in a game of Chess while for reincarnated Devils that have reached Ultimate-Class, they will receive the right to choose a part of the Underworld as their territory. There is actually no conflict when a Low-Class Devil starts his own group while still being a part of another peerage after becoming High-Class. When a Devil reaches High-Class, they receive a set of Evil Pieces to start their own group. However, even after going independent, Devils are still obliged to fight as underlings whenever their master has a Rating Game, for example, Issei still has to fight as Rias's Pawn if she ever has a Rating Game despite commanding his own peerage.

High-Class Devil Promotion Ceremony
As seen with Issei, when a Devil goes to their High-Class promotion ceremony at the ceremonial hall, that Devil and those with them are escorted in limousines under strict guard. Several security cars surround the limousines meaning the Devil becoming a High-Class Devil is treated as a VIP. At the ceremony itself, the Devil getting promoted stands in front of an altar (a reincarnated devil stands with their master) and the Satan in charge reads what is written on their certificate which states the reasons for their promotion. The Satan proclaims them as a High-Class Devil and they kneel down on one knee to receive their certificate with words of acceptance. They display their certificate to the audience to which they applaud. Next comes the crowning ritual. For a reincarnated Devil, they face their master at the altar, get down on one knee, and their master places a crown upon their head with another round of applause meaning the ritual is complete. For the final part, the Satan stands on the altar once again summoning a stone monument that shines in black which floats in the air. The monument is made from the same material as the Evil Pieces. The Devil getting promoted envelopes their hand in their aura and touches the monument. Their heart instantly makes a large beat and the monument shines crimson, showing their hand print for a moment and then the monument returns to normal. Their registration as a King is complete and they are handed a small box containing their 15 Evil Pieces for making their own peerage. As such, the ritual is done with the most crucial part of the ceremony. What comes after this is the words of congratulation from the guest which is read by the host of the ceremony and after the newly promoted High-Class Devil leaves the ceremonial hall, the ceremony is over.

Four Great Satans
The Four Great Satans (四大魔王, Yondai Maō) are the four rulers who control the Underworld. The current Four Satans are not the actual ones but were chosen to replace the four original Satans who died in the Great War, and each was named after the four original Satans: Lucifer, Beelzebub, Leviathan, and Asmodeus. Therefore the names "Lucifer" and "Beelzebub" are not personal names, but are more like managerial positions.

72 Pillars
The 72 Pillars (72柱, Nanajūni Hashira, referring to the 72 demons of the Ars Goetia) are a list of the 72 families of pure-blooded Devils (known as Pure Devils). Each clan possessing dozens of armies under them. After the Great War, however, every clan lost a majority of their armies and over half of clans from the 72 Pillars even faced extinction, with only 33 clans remaining (in reality, there are more than 33 houses that are still active but have lost some of their status in the 72 Pillars due to having mixed heritage).

Extra Demons
Extra Demons are the houses of top-class Devils who are not a part of the 72 Pillars. Currently most of the Extra Demons have secluded themselves from the current Devil Society, not wanting to involve themselves with the Government. Devils from the Extra Demons who involve themselves with the current Devil Government will have to break all ties with their households such as with the case of Kuisha Abaddon, Bedeze Abaddon (the 3rd Ranking Champion in Rating Game) and Roygun Belphegor (the 2nd Ranking Champion in Rating Games).

The only known houses are the House of Lucifuge (where Grayfia and Euclid comes from), the House of Abaddon (where Kuisha and Bedeze comes from) and the House of Belphegor (where Roygun comes from) Mephisto Pheles is the only Extra Demon that doesn't have a clan. A majority of the Extra Demons serve under one of original Satans, such as the House of Lucifuge serving the Satan, Lucifer. Rudiger Rosenkreutz revealed to be a Bishop from the House of Mammon.

Stray Devils
Stray Devils (はぐれ悪魔 Hagure Akuma) are Devils who have diverted away from their masters. Without their masters to keep their powers in check, they become a great threat if their powers go beyond their control. The Devils take this case very seriously, and any Devil who becomes a Stray will be killed on the spot. The only known Stray Devils in the series are Viser. Kuroka, the latter coming from the Vali Team and was the only known SS-Class Stray Devil as the Three Faction pardoned her of her past crimes.

Trivia

 * The families of the 72 Pillars are named after the 72 demons from the Ars Goetia, one of the five books of The Lesser Key of Solomon, also known as the Lemegeton.
 * Belphegor and Mammon are two of the devils associated with the Seven Deadly Sins along with Lucifer, Satan, Leviathan, Beelzebub, Asmodeus.
 * Abaddon sometimes appeared interchangeably, replacing Leviathan.
 * It is considered something of a rite of passage for a young up-and-coming Devil to make a pact with a Magician. Young Devils are warned to choose carefully when making such a pact as, if they choose the wrong Magician, it will reflect poorly on the Devil.
 * A Devil can have their Familiars hand out fliers instead of doing it themselves for Devil's jobs.
 * In Volume 8, Rias states that in the past Devils 'did' steal souls, however they no longer do such serious things.
 * In Volume 14, it was revealed that all the current Heads of the Devil households will leave everything to the next heir and enter their retirement after passing the household onto them.
 * The Japanese term for Devil (悪魔 - Akuma) is a bit of misnomer: the word 悪 (aku) means evil, cruel... However, not all devils are cruel and evil as the term stated. It could be assumed that the word 悪魔 is mostly used for reference or academic purposes.