Shortly after the Dawn War was won by the gods, a certain angel named Asmodeus was sent to the Elemental Chaos to guard an imprisoned being too powerful to slay. He was well trusted by the god he served, and quite powerful, as befit such a dangerous and important task. But while his strength was great, so, it seemed, was a weakness in his spirit.
While in the Chaos, he heard whispers of the great Shard of Evil in the deepness of the Abyss, the realm of demons. Though every tale spoken of it warned of its absolute power to corrupt, he heard only the promises of its power. Jealous of the gods he was forced to serve, and resentful of the task that kept him far from his beloved Astral Sea, he broke his vow, left his post, and traveled deep into the Abyss.
The travails within were as nothing to one of his might, and without delay he fought his way through swarms of demons, abyssal traps, and the very warped fabric of reality within that evil realm. Demon Princes arose and were destroyed, great planes traversed without pause. Ere the most powerful of demons could even realize what he was doing, he approached where few have ever dared tread, the black core of the Abyss where the Shard burrows.
With utmost arrogance, he took this final step and became one of the first beings ever to see the Shard with his own eyes; what form it took and what specifically transpired is a tale only that Horned One can tell. What is known is that as he began taking the power of the Shard, the Abyss itself rocked from the impact of what he was doing. Unified in a rare moment of desparate self-preservation, the remaining Demon Princes banded together and fought their way to where Asmodeus was in the process of stealing the force that sustained them. Even he could not take on their combined might, and he had to flee.
Though they prevented him from stealing the entire shard, he nevertheless left the Abyss without empty hands; he bore a Splinter, a fraction of that evil power in the form of a Ruby Rod that swelled his might to impossible levels. Bearing this weapon, he arose through the cosmos and approached the domain of his betrayed master.
The battles were brutal, but not long. Asmodeus, and the other angels he was able to tempt to his side in that first mad rush, slaughtered the servants who stayed true to their own divine master. He himself faced Asmodeus, but was slain by his former servant wielding the power of the Ruby Rod. Asmodeus killed him so completely, even this god’s name has been ripped from the history of the world, and he can be referred to now only as He Who Is Not Named.
As Asmodeus took up reign in his new world, it changed to reflect his corrupt soul; the domain became a place of torment and evil, and Asmodeus’s followers became devils, rather than angels. He himself expected godhood, but even with the Rod his power was not that strong, and he bitterly raged against the fact that he was no more than the most powerful devil himself.
For reasons known only to themselves, the gods did not oust this usurper. It is claimed that they required a threat against their mortal followers; a punishment if the failed to obey the edicts of their god, and so the Codex was written and signed in divine blood. What exactly the Codex enforces upon both sides is unknown by any mortal, nor it is known what punishment awaits either god or devil who breaks their word. What is widely suspected is that Admodeus was granted the domain he took in conquest, and that no god would contest his rule. While short on his own followers, he would be given the souls of those who died while damned by the god they worshipped. No one is fool enough to believe that Asmodeus is content with what he holds, but whether held in check by the threat of annihilation at the hands of the combined divine brotherhood, or just playing a very, very long game, he has never directly worked against his oaths in the Codex.
As for the demons, they have loathed the devils ever since the event that created them. They long to steal back the Ruby Rod and return the power stolen from their Shard, while Asmodeus lusts for the rest of the power he was forced to abandon. This feud has raged since that day, and has been dubbed the Blood War. At the moment, the war is in a state of seige as both sides regroup and plan their next move, but any alliance is unthinkable. Any devil in the presence of and demon will immediately attack, and vice versa. They each constantly watch for ways to thwart each other at every turn.