The Devil (from Greek: διάβολος or diábolos = 'slanderer' or 'accuser') is believed in many religions and cultures to be a powerful, supernatural entity that is the personification of evil and the enemy of God and humankind. The nature of the role varies greatly. It ranges from being an effective opposite force to the creator god at one extreme, where both are locked in an eons long holy war for human souls on what may seem even terms (to the point of dualistic ditheism/bitheism), to being just a comical figure of fun or even an abstract aspect of the individual human condition at the other.
Whilst mainstream Judaism contains no overt concept of a devil, Christianity and Islam have variously regarded the Devil as a rebellious fallen angel or demon that tempts humans to sin, if not commit evil deeds himself. In these religions – particularly during periods of division or external threat – the Devil has assumed more of a dualistic status commonly associated with heretics, infidels, and other unbelievers. As such, the Devil is seen as an allegory that represents a crisis of faith, individualism, free will, wisdom and enlightenment.
In mainstream Christianity, God and the Devil are usually portrayed as fighting over the souls of humans, with the Devil seeking to lure people away from God and into Hell. The Devil commands a force of evil spirits, commonly known as demons. The Hebrew Bible (or Old Testament) describes the Adversary (Ha-satan) as an angel who instigates tests upon humankind. Many other religions have a trickster or tempter figure that is similar to the Devil. Modern conceptions of the Devil include the concept that it symbolizes humans' own lower nature or sinfulness.
People often put the concept of the Devil to use in social and political conflicts, claiming that their opponents are influenced by the Devil or even willingly supporting the Devil. In addition, the Devil has also been used to explain why others hold beliefs that are considered to be false and ungodly.
All Devils have increased physical abilities, such as superhuman strength and endurance, and 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. All Devils have a unique ability called "Language", which allows them to communicate with different people from around the world without the language barriers. According to Rias in Volume 4 of the light novels, Devils have an immunity towards getting sunburned.
High-Class Devils that have a "peerage", 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.
It is stated in one instance that 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 unique to that clan, though occasionally descendants of those who marry out of the clan will also inherit these abilities. Examples include the Phenex's power of immortality, the Bael's Power of Destruction, and the Abaddon's Power of Hole. The origins of these powers are presently unknown. The pure-blooded Devils who do not 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 Bilingual 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.