Long ago, Gods used to walk the land of Zerynthia. There are twelve Gods ― four Construction Gods, three Blessing Gods, three Special Gods, and two Chief Gods. The Chief Gods were the ones who ruled over the other gods, and they were Pólemos, the God of War, and Zoe, the Goddess of Species. As history says, the Republic of Ein was the first country that united Zerynthia, led by the Chief Gods.The two Gods then divided its territory into four regions, three of which are now the nations of: Alovetta, led by the Construction Gods; Synestia, led by the Blessing Gods; and Neith, led by the Special Gods. Eldenbaum was the central government and the capital of the Republic of Ein, becoming a prosperous and advanced nation. The God of War then further assigned five great noble houses to rule over the cities near Eldenbaum.However, this prosperity finally came to an end. The God of War suddenly ignited a war against other nations by initiating an attack on Neith. The God of War then ordered the five great noble houses to send their men to aid him and the imperial soldier in the war.The five great noble houses refused ― they did not find any reason why Ein should attack other nations and break the peace they'd been keeping so far, questioned the reason behind the God of War's attack, finding his order as a form of absurdity. Due to the five great noble houses' defiance, they were captured and sacrificed to the Carronoah, a weapon of mass destruction created by the God of War and the Goddess of Species, acting as the 'projectiles.'

A hunt within the capital was then conducted ― citizens of Eldenbaum were captured and made as Carronoah sacrifices. In the meantime, the God of War began to attack other countries with the Carronoah, powered by human sacrifice souls and negative energy ― called the Hex.War finally broke out between the four nations. Countless innocent citizens and bystanders died while caught in the heat of war, but the tide was turned against the one who ignited the flame of battle. The Carronoah self-destructed as the result of overspilling the Hex inside it, creating a massive explosion that decimated the entirety of Eldenbaum and the citizens who tried to escape.This also resulted in the death of the God of War and the Goddess of Species. The two Gods' death did not mean that their bloodline stopped there ― before they died, the God of War and Goddess of Species had produced an offspring, a living legacy of the two forgotten Gods, who now resided in the neighboring country.The other Gods finally decided to seal Eldenbaum to prevent the poisonous Hex from the Carronoah from reaching their respective regions.Eldenbaum became a deserted city from its destruction and sealing as the aftermath of the very first war before the Zerynthian calendar. Undead spirits of the sacrificed people to the Carronoah and victims to the Carronoah began to wander aimlessly and mindlessly throughout the city, alongside various monsters unknown to men. From that day on, the Republic of Ein was now known as the Dark Domain of Ein and became forgotten by mankind as the Gods erased every trace of Ein from every Zerynthian’s memories.

Demons are supernatural beings that formed from and fed off human malice and negative thoughts. No one knows for sure how they were created, but demons have inhabited the underworld for a very long time. While living in the underworld, they can’t go up to the human world by themselves. Someone from the surface needs to open a gate for them to pass through, one of the ways was by forming a contract with them. Demons are recognized for several features they have, such as a pair of horns, wings, noticeable sharp canines and claws, and tails. Some of them can also take a monstrous form, like having multiple body parts.Demons are divided into three levels: Upper-ranking, Middle-ranking, and Lower-ranking. Lower-ranking demons consist of mindless, animal-like demons, and are the most common demon that roams in the Dark Domain of Ein. They are also the weakest of all of the demon, but don't be mistaken ― even the weakest demon is still tougher by human standards.ㅤㅤMiddle-ranking demons are humanoid ― albeit some of them possess distorted body proportions and monstrous facial features ― demons that are tougher than lower-level demons. They are proficient in wielding weapons, possessing enhanced strength and speed than an ordinary human, and unlike lower-level demons, they have regenerative abilities that allow them to continue to fight even in a tattered condition.

Middle-ranking demons are also bloodthirsty and barbaric, always looking for carnage even among their demon peers. Middle and lower-ranking demons can be slain by casting holy magic or performing an exorcism.Upper-ranking demons are the strongest, most intelligent, and most dangerous demons. They are much fiercer and more cunning than middle and lower-ranking demons, and among all of the demons, they have the most human-like appearance. Their regenerative abilities are much stronger and faster, as they can quickly replace their injured or missing body parts in mere seconds. Upper-ranking demons also possess several strong supernatural techniques that they use in combat. Unlike the middle and lower-ranking demons, upper-ranking demons cannot be killed by only casting holy magic. Holy magic can only weaken them for a bit ― instead, their weakness lies in their heart.Once their heart is destroyed, an upper-ranking demon can finally be slain. Upper-ranking demons always keep their heart unexposed ― and the only way to expose their heart is by cutting off their head. However, beheading an upper-ranking demon is not as easy as it seems.An upper-ranking demon can be distinguished from other demons by particular markings bestowed by the Demon King on their body.

One can form a contract with a demon by offering their blood and one of their body parts as a 'guarantee' for their contract with the demon. If the contractor dies, the contract can be passed down to their offspring. Conditions of a demonic contract are:- The human contractor can order the demon bound by their contract to do whatever they want, and the demon must obey their order.
- When the first contractor dies and their demonic contract is passed down to their offspring, the demon also takes the same body part as the first contractor’s offering from their offspring, as an assurance for their contract.
- When the contract has been fulfilled or deemed unworthy and not beneficial for the demon, the demon will end the contract by feeding off the human contractor’s soul.

Once the contract has been finalized between the two sides, a seal of demon will appear on the human contractor's body as the sign of the contract.

ㅤㅤApproximately a thousand years after that, Yohm, the ninth emperor of Alovetta, escaped to Ein after a coup d'etat broke out. The coup d'etat was led by the then-tenth empress of Alovetta, Erza, and resulted in his exile from the Holy Empire of Alovetta. Filled with rage and desire for revenge, Yohm formed a contract with the demons to wage a war against Alovetta—thus opening a way for them to come up to the surface with the Gate of Qliphoth.ㅤㅤFrom that day on, Eldenbaum became a city inhabited by demons, and demons quickly took control of the undead and monsters contained there.ㅤㅤThe Demon King, ████ █████, formed an army of demons and undead spirits under his rule. The Demon King desired to conquer the other regions while fulfilling his contract with Yohm to aid him in war with Alovetta after the seal around Eldenbaum was released.ㅤㅤAfter repairing the High Palace of Eldenbaum within one night with his demonic power, the Demon King began to rebuild the entirety of the fallen capital.ㅤㅤThree years later, with the undead spirits and monsters doing the labor work under his orders, the Demon King established a colosseum made from the remains of decimated house buildings. Inside this colosseum, a tournament was held ― a tournament where demons fight and fight, tearing, and mangling each other gleefully as a form of exercise and entertainment.

ㅤㅤAfter a long and bloody battle, a single demon emerged as the victor of the tournament. A young demon who had a history with this cursed nation. Many demons were surprised by this young demon's capability, for he was not originally a demon ― he was once a human who inhabited this capital, before getting his life taken by the greed of the Gods. The demon boy, who looked harmless and detached on the outside, hides unwavering anger and hatred inside of him.ㅤㅤAlthough his victory was already expected ― the markings across his mien are enough to tell the other demons that this young demon had received the Demon King's blessing, even before the tournament was held. As a prize of the young demon's victory over dozens of his kind, the Demon King crowned him as the first Apollyon ― a soon-to-be group of upper-ranking demons who were just below the Demon King in terms of power.ㅤㅤSeveral tournaments were held years and years later, although not all of the victors are blessed enough to receive the Demon King's grace to be an Apollyon. In the end, there is finally six appointed Apollyon, who then act as war commanders that lead the demon army.