In the world of Highschool DxD, there exist several other supernatural species besides humans. (And for the purpose of this, it can be assumed they also roam Nexus likewise, or maybe Nexus even is the world where all is set in.)
Devils are one of the major three factions, the others being Angels who serve (the Christian) God, and Fallen Angels, who descended from that path and became largely independant. There are others, like the Norse Gods residing in Valhalla and many more, but most of these have less influence and power. Hellish and heavenly plains exist, the devils inhabiting hell, while the angels reside in heaven, and the fallen angels mixing into societies here and there, also frequently appearing in the normal human world.
It is highly unlikely and unusual for devils to visit heaven, and vice versa with angels, but they often enter the human world or even (at least temporarily) live there. In old times, the numbers of angels and devils had been way greater, but turmoil and war broke loose, neither side gaining a crucial advantage over the others. The bloodshed was finally stopped, as all factions were too thinned out to keep harnessing old grudges, and now a period of stability and peace exists, although sometimes the harmony is frail.
While theoretically at peace, a treaty having been signed and the powers even forming alliances against outer threats sometimes, now and then fights break out between isolated individuals or smaller groups. An all out war is highly unlikely though, and each side respects the others in some way, and leaves them be plus doesn't muddle with their problems, followers or possessions and locations.
There are two types of devils, the rare pure-blooded devils mainly coming from the few remaining powerful families, and the hybrid beings known as 'reincarnated devils'. Devils with sufficient abilities and enough reknown as well as status are granted permission to create subjects / underlings of mortals, bringing them under their direct supervision and starting a 'peerage'. They are responsible for them from that point on, but can of course use them for their needs and give them orders. The method was invented to fill up the ranks which were severely depleted after the great war, as devils reproduce extremly slowly, even one child every 100 years is already considered extraordinary. (Angels supposedly developed a similar method to create 'reincarnated Angels'.)
All devils, even reincarnated ones, have an extraordinary long lifespan, bordering on immortality unless special, unnatural circumstances occur. Once they reach adulthood, their physical appearance remains almost the same no matter what age they reach, devils appearing youthful, handsome and pretty even when millenia old.
In modern days, devils (and angels, for that instance) try their best to keep knowledge about them hidden, mingling with the normal society to pursue their goals. To gain influence and generate income, devils make contracts with the few mortals who know of their existance, offering their help (which can be anything from trivial things like helping in the household or go shopping to more difficult and dangerous tasks like handling cursed artifacts or fighting enemies). They are 'summoned' with the use of a drawn, magical summoning circle, though it is also possible for devils to hand out prepared 'calling cards' allowing their clients to directly contact them with that means, without the need to do the ritual themselves.
The flyer that Rias' group hands out looks like
The devil society knows four different
ranks, which just tells something about the social status and isn't necessarily equalling power:
Ultimate-class Devils have the highest position in society, and it's very rare for someone to be granted access to this class at birth. As a devil comes into the class, they gain a rank in the underworld as Satans, Great King, King, Archduke, Archduchess, Duke, Duchess, Prince and Princess.
High-Class Devils is a class typically consisting of the majority of younger (pure-blooded) devils who have yet to master their power. Includes Marquis, Marchioness, Margrave, Margravine, Count, Countess, Viscount, Viscountess, Baron, and Baroness.
Middle-Class Devils incorporate the middle class of new devils who have passed one of the three raise ranking. Includes Baron, Baroness, Knight, and Dame.
Low-Class Devils form the weakest class who are usually reincarnated Devils or are just commoners. They are a part of a High Class Devil's peerage and serve one of the higher class. Low-Class Devils can promote themselves to higher rankings depending on their performance.
There are three different ways to raise one's ranking:
Contracting: Doing Devil Jobs by granting the client's wish. This includes having a high number of contracts completed and/or receiving high quality rewards. Devil contracts will garner points over time, but it is a gradual process unless the human customer is someone with great power and influence (such as a president or leader of a country). This is considered to be the slowest method; also, a promotion through contracts is seen as more of a tradition.
Military Accomplishments: Obtaining great achievements in times of crisis or situations. This includes showing exceptional ability or skill in combat (rare even before the Kuoh Treaty as everybody feared mutual annihilation if another war broke out). By defeating high ranked enemies of the Devils, such as leaders of the Angels or Fallen Angels, a Devil can be rewarded. This is the fastest method, however, this route is unlikely in the current stalemate without a full-scale war.
The Rating Games: The last method is by doing well in Rating Games. This includes earning high points, public esteem, and opinion in official Rating Games. This is the modern path for ascent in Devil society, as it was designed to promote and train new powerful Devils. The Rating Game allows reborn Devils to hone and show off their skills and abilities against others in a controlled non-lethal environment. Performing well in the Rating Game raises reputation and leads to promotion. In fact, the Top 10 rankers in the Rating Game have all reached Ultimate-Class, with the 7th ranker being the highest ranked reincarnated Devil. This is by far the best method to raise one's own 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, if Issei were to reach High-Class and start his own peerage, he would still have to fight as Rias' Pawn if she ever has a Rating Game.
Speaking of chess and Evil pieces. The devils have designed this way to have a more standard, formalised way of high-class (or above) devils gaining retainers. Each devil can have up to 15 followers, the different positions equalling chess pieces. The leader is the 'King' (even if the devil is a female one), who can get to command a queen, two rooks, two knights, two bishops and eight pawns. Devils who gained the right to start a peerage are given the fifteen other pieces. The magical figurines, in combination with a demonic power, allows them to turn someone into a reincarnated devil under their leadership, if they accept it - the subject in question has to agree to the process willingly. This can even be used to save someone who is on the brink of death. It is not needed to collect what is known as 'full set' (so king plus all 15 others filled up), and the abilities vary from one person to the next, with some exceptionally gifted individuals requiring more than one piece used (of the same kind, though) to have the ritual succeed. Sometimes, a piece is regarded as 'mutation piece', where one potential subject who
should use up more than one piece ends up requiring only one after all, this only happens very rarely though.
King: Leader of the peerage, as someone has to be ranked at least a high-class devil, they almost always have a number of strong abilities, and can use several special devil powers.
Queen: This position combines the advantages of all other pieces. Physical durability and enhanced strength is coupled with great speed, plus very often queens boast a high magical power. Nigh exclusively, they are the second in command of the group. For rating games, the queen is worth 9 pawns.
Rook: Rooks have vastly increased physical power, mainly strength and sturdyness. They have both a high offense and defense, though they are relatively slow. A rook can make a 'castling' move with the king. A rook is worth 5 pawns.
Knight: Knights are very quick and have fast reflexes, plus they tend to yield swords with which they have a good combat prowess. Their agility allows them to use manoeuvring and attacks at high speed, but they have a low durability if something manages to hit them. Also, having their legs injured almost renders them useless in a fight. They are worth 3 pawns.
Bishop: A bishop typically has only low physical powers. They are adept magic users though, the exact details varying. Some are supporters and healers, while others also utilise offensive spells. If they use high power abilities too often, their magical energy runs low, leaving them vulnerable. Like knights, they are worth 3 pawns.
Pawn: The foot soldiers, normally without any special abilities and only moderate combat abilities. They make up for in number what they lack in individual power, though this greatly varies from one pawn to the next. They have one special ability though, promotion - should they manage to enter an enemy territory (i.e. the enemy base in rating games), then they can power up by taking over the traits or another class (except king). Not surprisingly, a pawn is worth exactly 1 pawn, duh.
All devils have a number of innate abilities and traits. Compared to humans, they have an improved physique (no matter which piece they equal, though the different pieces further diversify and specialise devils), and more accurate senses. Even in almost darkness, they can see a bit, and they can focus their hearing to listen to a conversation held several dozen meters from them. Sunlight doesn't harm devils, though high concentrations of light seem to cause some unease - and the church as well as some fallen angels have been known to use weapons of extreme light concentrations, which can severely hurt devils. Going near a church, seeing holy symbols or doing acts such as praying causes them to feel a bit ill, inflicting headaches and other things. At will, devils can make their demonic features appear, a set of two leathery, bat-like wings sprouting forth. It allows them to fly for a certain amount of time. Devils also seem to have an increased healing speed, their bodies slowly able to regerate wounds, this can be increased by high-class devils sharing some of their power with a recipient, even wounds which would be fatal for mortals disppearing overnight. A number of devils also seem to have vorish abilities, although somehow it seems to be mainly the female ones who display these traits, and can nicely use their stretchy, flexible bodies and a potent digestive tract to handle live and person-sized prey.
Devils are able to twist their language, so that nearly anyone can understand what they say (without speaking their language themselves), and skilled enough devils can read or even change the minds of mortals and lesser beings. They can also have special servants called familiars - on the night of a full Moon, Devils may go to a special forest and, with the guidance of a Familiar Master, capture and bind a familiar they find suitable. It can range from small (albeit sentinent) animals, to other supernatural creatures, demi humans etc, and most can also shapechange. Familiars can take care of mundane tasks such as gathering information, handing out fliers and the like, but in theory any task immaginable could be given to them.
Using a magic ritual which incorporates the same circle that is used for summoning, devils can also teleport to other locations. It takes some preparation, and the user has to have some magical skill at least. Someone with a too little grasp on demonic powers can't use this method by themselves, although a group teleportation is possible - however, only members of one peerage can participate in it.
Two more notable things exist: Sacred Gears and special weapons, both Holy Swords and Demon Swords. Holy or demonic weapons are high-quality items of immense power, though not everyone can just pick them up at leisure. Unless you are somehow directly 'compatible', anyone hoping to yield a Holy Sword has to train and undergo mental exercises until they manage to field the blade in combat (and even then, success is not guaranteed). The church has invented chrystals to help with it, which serve as catalysts by emitting energy patterns resembling some who already can yield Holy Swords, but even that is no guarantee. They are powerful weapons and cause high damage to devils.
Demon Swords are pretty much the opposite, they also can't just be used by anyone, and are formidable against angels and fallen angels, as they emit demonic energy. Sacred Gears come in many shapes and with many different functions. Some mortals seem to be bestowed with them - by fate or just coincidence (only humans or demi humans are born with them) - and their yielders can slowly learn to master them. Each Sacred Gear is completely unique, and has special abilities, which are always helpful in a way though. After being reincarnated as a devil, Sacred Gear users keep their abilities, and someone who can yield special weapons can do that even after the transformation (despite devils normally fearing Holy Swords, for example). While the basic abilities and the unique features always show, it depends on the willpower and mental focus of the user how powerful a sacred gear can become.
Should one unlock the most powerful moves, the technique is called 'Balance Breaker', as it's said to be like a 'bug in the system' either overlooked or purposefully created by God, which can shift the balance of powers. It typically requires advanced training, high skill with the sacred gear and often needs a *trigger* for the balance breaker to happen - a great emotional turmoil like the loss of a close friend or loved one, entering an extremly angered / livid state or extreme, life-threatening danger.
