Yeah I know this isn't my primary story and I really should be updating that instead of this but I've had this idea in my head for a while now. So I hope you enjoy this.
In recorded history no war has ever been as destructive and bloody than that of the War of the Three Factions. Better known as the Great War the factions of Heaven, Hell, and Fallen waged a war against each other for supremacy. Heaven, lead by the God of Christianity, wished for a peaceful world where humanity could live in service to God. Hell, lead by none other than Helel or better known as Lucifer, wanted revenge and domination. Revenge for, in his eyes, being wrongfully cast down from Heaven and to rule Humanity by fueling their desires and sins. The Fallen were lead by Azazel, a relatively young Angel but among the first to fall. Azazel just wished for a world where the Fallen could live without fear of death. Heaven shunned them and the denizens of Hell hated them. The only place that they could be was the Earth. However, the two other factions took this as a sign of dominance. A sign that the Fallen were going to take over the Earth for themselves. God was able to keep Heaven from taking action against the Fallen but Lucifer saw an opportunity in this. Lucifer struck out against the Fallen being the spark to ignite the flames of war. The Fallen had no choice but to retaliate against Lucifer's unprovoked attack and Heaven then had to intervene. God may have cast down His children but he still cared for them and did not like seeing his children slaughtering one another. It was with Heaven's interference that caused the other two factions to form a temporary truce. Neither side could take on Heaven's forces while fighting each other.
With the Fallen and Devils temporarily aligned the war took a drastic turn, Heaven started losing ground and it seemed as if Heaven would soon fall to the combined might of the two other factions. However, Lucifer turned on his Fallen allies and chaos returned to the world. To this day there wasn't a being alive that knew the reason Lucifer would make such a decision when they were so close to victory. Some believe that it was the Morning Star's hubris, and others believe that he was striking preemptively against an attack he believed to be incoming. It matters little though because for several hundred years bloody conflict scarred the world. However on one fateful day the Heavenly Dragons started to rampage through the battlegrounds of Heaven. The three factions had to momentarily cease their fights in order to defeat the two behemoths. For the first time in millennia Lucifer and Michael fought side by side. Kokabiel and Zekram Bael, two bitter enemies, fought alongside each other as if they had been allies for centuries. It was only with their combined might that the Factions were able to slay the dueling dragons and then God sealed them away into the Sacred Gear system.
The following month lead to a tentative ceasefire as the Factions recovered from their losses. Heaven lost several Seraphim, the Fallen lost many of their forces and Azazel pulled them from the war, and the Devils lost both Leviathan and Asmodeus. Battles between the Devils and Heaven resumed after the short respite. For another full century the two sides battled until in one fateful battle Lucifer and God both perished. Nobody saw the fight, however when the two bodies fell from the sky landing in the middle of the battlefield the entire battlefield became silent. It was after this the the two sides declared a more stable ceasefire. Beelzebub and Archangel Michael both agreed that their respective factions could not survive another battle. Azazel also agreed to the ceasefire even with Kokabiel's demands to start the war anew.
When Beelzebub returned to the Underworld the seeds of unrest began to lay root. Some of the lower houses of Ars Goetia saw this as their opportunity to rise in status, and younger devils wanted to change the system. They wanted to live in harmony with other races chief among them being humanity. Half of the Ars Goetia rallied around five supremely talented but young devils. Sirzechs Gremory, son of Venelana Bael and Zeoticus Gremory. Sirzechs is gifted with precise control and unprecedented power over the Bael family's Power of Destruction. Serafall Sitri, the Unbeatable Princess of Ice. Falbium Glasya-Labolas, a man who embodies the sin of sloth and yet is the greatest militaristic mind the Underworld has ever seen. Ajuka Astaroth, the perfect balance between immense demonic power and the mind of the world's most renowned scientist. Lastly is Azras Marchosias, a tall onyx haired man. Azras is the foremost expert in the Underworld on all things magic and his familial trait takes that knowledge to the next level. The ability of transmogrification is a terrifying ability when mixed with an intimate knowledge on the very foundations of magic.
Azras is pacing in the commander's tent looking over a map of the battlefield. He is dressed in simple light armor. The armor is a dull black with golden accents with a cape flowing behind him. A soft sigh leaves his lips as he sits down and buries his face in his hands. Being so engrossed in his thoughts Azras did not notice the person approaching from behind him. The person places a hand on his back causing him to stiffen but after a moment a smile dances across his face as he realizes who it was. Azras turns around and his eyes are immediately met with two orbs of bright blue. Long black hair done up in twin-tails with pink ribbons and a mirthful smile on her face. "It's good to see you Serafall, I wasn't expecting you to be here this early."
A soft giggle emanates from Serafall's mouth, a sound that he loved hearing more than anything else, "Well I knew you would be here by yourself and I didn't want you to be alone." She leans in and whispers in his ear "My bed was so cold without you in it." A shiver goes down Azras' spine as the woman's hand works its way into his armor and starts rubbing across his chest. "Well someone has to be the responsible one. We both know Falbium wouldn't be up this early in the morning."
Azras leans in and gives a Serafall a gentle kiss deciding to allow her this small bit of intimacy for but a moment. He gently pulls back causing the Sitri heiress to pout and whimper at the sudden end to the kiss. She quickly pulls him close and captures his lips in a rather heated kiss which Azras quickly melts into. His arms wrap around her and pull her tight against his chest they are so into their kiss that they don't stop until they hear a cough emanate from behind them. The two young devils quickly part from each other as they both send a half-hearted glare to the crimson haired man that interrupted them. Clad in armor very similar to Azras' own was a man of similar height with red hair cascading down to his shoulders. A rounded face and blue green eyes greet them with a small smile, this is none other than Sirzechs Gremory the New Devil Faction's trump card and leader. The mirthful smile never leaves his face as he goes to the table and sits on the opposite side of them. "I'm glad to see the two of you are in high spirits." After a few moments his face turns more serious "What do you think of our current situation Azras?"
Azras sits down across from Sirzechs, his eyes roaming the map "We are in an advantageous situation. With the Lucifuge turncoat our firepower has increased tremendously and we took the capital of Lilith with relative ease. All that remains to finish this bloody war is to raid the Beelzebub compound and either get Beelzebub to submit or kill him. Our situation is not perfect however, at any moment one of the other factions could come and attack us while we are weakened. The demons have also been restless as of late and I fear that they will attempt to take our half of the Underworld. That is a war we would be unable to survive without assistance. I suggest we get into contact with the Fallen to form a temporary alliance."
Sirzechs raises an eyebrow at Azras' suggestion the Fallen and Devils hated each other and an alliance seems unlikely. "What leads you to believe that the Fallen would come to assist us? We have spent the last several hundred years in a blood war with each other." Azras nods once before replying to his brother in arms "They would assist us out of necessity. More than anything else the Fallen need land to call their own. Should the Demons start an assault against us with the Fallen's help we can defeat them handily, and as for compensation we allow the Fallen to take over the Demon's half of the Underworld. As for the Demons we lock them into Hell, as of now only the Slavic pantheon and the first vampire are locked within Hell's gates. There is plenty of room there and the Marchosias family was entrusted with the spell to summon the gates, so we do have the option should the need arise."
Sirzechs gives Azras a resolute nod "Hopefully it will not come to that but it never hurts to be prepared. Let us look to our current foe Beelzebub, what do you believe is our best option in dealing with him?" Azras moves some of the pieces on the map so that the entire army is surrounding the compound "My move would be to take the entirety of our forces and completely crush the compound. Level it to the ground, killing each and every devil inside of it." Azras notices the glare Sirzechs leveled to him and continues, "However I know that you are going to be very much against that plan so we won't go with that idea. Instead we should take half of our forces and surround the castle. After that we send in a small group to flush Beelzebub out where you can take him on one on one. As for who should be in the group I would say that the best people for it would be Ajuka, Diehauser and myself. The three of us are powerful enough to flush him out, but can also keep the collateral destruction to a minimum."
The crimson haired devil gives a resolute nod to the plan "We shall strike tonight then. We need to end this war as soon as possible. Our race has seen far too much bloodshed as it is and I don't intend to allow it to last much longer." The two other devils nod once in agreement ready to finally be done with this bloody war.
The plan worked to perfection, the group of Diehauser Belial, Ajuka, and Azras were able to efficiently flush out Beelzebub. They suffered little more than scrapes and bruises in the battle and Sirzechs was able to easily overpower the older devil. Sirzechs' power of destruction completely erased any signs of the now dead Maou. With Beelzebub now dead the Old Maou supporters have nobody to rally around especially with the head of the Lucifer house fleeing the Underworld. In the following week Zekram Bael nominated five candidates for the position of Maou. Those five being the group of Serafall, Ajuka, Falbium, Sirzechs and Azras. Everything seemed to be going well and that is when the Demons attacked. They started their assault on the capital the night of the confirmation of the new Maou. However, Azras had planned for this and had been in contact with Azazel since the death of Beelzebub. It took little more than a hour for the Fallen army to aid in the battle. With the combined forces they were able to repel the invading force back into their half of the Underworld. It was at this point war began anew. However this time it was more like slaughter as the Demons stood no chance against the coalition of Devils and Fallen. Genocide was not on the table though, that was something agreed upon by the Devils and Fallen. Both sides wished to be better than they were before and as such Azras' plan of locking them in Hell was put into motion. Azras locked himself in his family's library pouring over ancient texts to get the ritual to summon the Gates of Hell memorized. Within a few months Azras succeeded in learning the spell to open Hell. There is just one problem with this however, the spell requires a sacrifice. The spell forces the user to enter the realm of Hell as well. Azras knows that this is not an elegant solution but it is a quick one. An argument he has been having with his fellow Maou. Obviously Serafall was incredibly against this as was Sirzechs. But Falbium and Ajuka agreed that this was their best option, even if it was an option that was taxing on them personally. After several days of arguing the group relented and would allow Azras to make his sacrifice. Azras entered the frontlines standing tall and proud. His long black hair flowing in the wind behind him. His torso was unclothed and you can clearly see the magical runes coating his chest. Azras begins to chant and two massive doors appear from the ground. Skulls and chains adorn the massive structure and chains lash out attaching to every demon and Azras. A small smirk appears on Azras' face and just before he was violently pulled into the doors along with the demons he says "Step one is complete"