Disclaimer: I do not own both Highschool DxD and Harry Potter as they are owned by Ishibumi Ichiei and J.K. Rowling respectively.

The story will start during the world war II in pre-canon Potterverse.

Issei will not be a cliche hero that will save everyone. Expect OP, OOC Issei.

There are OCs

Prologue

~Issei POV~

22 Years... 22 years has already passed since the defeat of Khaos Brigade. It feels like it was just yesterday that I was reincarnated as a Devil...

22 years ago we were hard-pressed by Qlippoh, Indra, Hades and the Hero Faction. All four of them allied with different reasons but with the same goal... our destruction.

Qlippoh wants to use Trihexia to defeat Great Red and open the portal to the other world, Indra wants to defeat Shiva to rule the Indo Pantheon, Hades wishes to free the Titan from Tartarus and use them to conquer the Underworld, and the Hero Faction sought to kill all who stand in the way of Humanity's 'Ascension'.

Various Factions like the Norse, Greek, and Magicians has helped but the enemy was too strong and in result there was a frail power balance that can easily be toppled.

Wishing to break the stalemate, the enemies invaded our home, Underworld, Valhalla, Olympus, and Heaven, at the same time. The invasion on Valhalla and Olympus was easily repelled but Underworld and Heaven were in ruins. Only then that we realized that the invasion on Valhalla and Olympus was a diversion to prevent Greek and Norse intervention. As a result Falbium Asmodeus-sama and Archangel Metatron and Raguel died. After investigation, it was revealed that the barrier of Heaven and the defenses of Underworld was disabled by spies.

Instead of lowering our morale, they served as martyrs. The neutral was inspired and everyone doubled their efforts to root out the spies. Slowly but surely, the enemy was being pushed back. Hades and the few Titans that were freed was defeated first but not without a high number of casualty, after that Indra and the Hero Faction was defeated. Only Qlippoh and the Stray magicians remained.

Our heightened effort turned out to be fruitful as the traitors were killed and lost territories were annexed back. After the execution of most traditional devils, high ranking angels and high Grigori member that were proven to be traitors, the remaining spies that managed to escape our notice stayed low or fled in fear.

Qlippoh stayed off the grid for a time, gathering the remnants of our old enemies. And then with aid of the grim reapers, Indra's army remnants, and a few former Heroes they attacked the rating game used to heighten the morale. The whole artificial dimension used as a battlefield was destroyed and Qlippoh tried to unseal Trihexia in the Dimensional Gap.

Despite our efforts, we failed to stop Qlippoh and the six hundred and sixty-six seals that sealed Trihexia was destroyed.

Nilrem and Hexxenacht was defeated but Walburga disappeared. The evil dragons and except Crom Cruach was killed. We attempted to defeat them but more pressing matters in form of Trihexia diverted our attention.

The Qlippoh then teleported everyone including Trihexia to the City of Lilith, the Devil Capital. A lot died, men, women, children none were spared from Trihexia's curse. Distracted by Trihexia, I was caught off guard by Rizevim. But fortunately or rather unfortunately my dear, Rias, took the attack meant for me…

I tried to deny it, I cradled her in my arms and tried to heal her with Phenex tears but the aura of Trihexia was nullifying it. Tears were in my eyes, Rias smiled upon me and said 'shhh... I'ts okay my dear Issei... It's not your fault... and always remember that.. I Love You...' her eyes then closed and they never opened again.

In a fit of rage Sirzech-sama tried to attack Rizevim but Lilith intervenes. While Lilith and Sirzech were fighting, I was staring blankly at my beloved's corpse. By that time, I was angry- no scratch that, I was enraged- no… the best word that can describe me that time was Furious.

《Cardinal Crimson King Juggernaut Drive》, was the name of the state I was, another form I unlocked due to my rage. With Lilith being busy with Sirzech-sama and Crom Cruach fighting Vali, I engaged Rizevim who only laughed and taunted me with my beloved's death.

Even though he was hailed as a super devil, he was no match for me in my state of madness. I overpowered his cancelling ability by delivering overwhelming amount of force in short amount of time and ripped his wings, severed his limb and only then that he, for the first time, showed fear... and laughed hollowly as called me a monsters befitting of a devil...

Meanwhile, Sirzech-sama proved his worth by temporarily binding Lilith but not without taking a fatal damage but that was enough for Ophis to absorb her clone. Ophis then tried to engage Trihexia in a battle but is being pushed back because she was still weaker than normal and she tried to limit the damage done to the city.

Crom Cruach then retreated after beating Vali. Before leaving, he expressed his eagerness to fight me at my top state.

When I was still filled with rage, I attacked Trihexia and tried to defeat it but to no avail. Even with the 《Cardinal Crimson King Juggernaut Drive》, Trihexia was still at another level so I was easily defeated.

At this time, Asia was helping the injured but was devastated when she learned that the curse of Trihexia can nullify her Sacred Gear. So she filled herself with resolve and determination to heal and used the 《Holy Twilight Prayer》to sacrificed her life force to heal all she deemed as allies.

Meanwhile, in the brink of defeat, I was pulled to my soulscape by the 《True Longinus》, which chose me as its new wielder after Cao Cao defeat but was inactive until now. There an essence of Biblical God appeared, he snapped me out of my mad state and asked me to use the 《True Longinus》 to reseal Trihexia once and for all.

After being awakened from my mad state, I transformed into 《Cardinal Crimson Crusader Dragon God》 which is superior more than the 《Cardinal Crimson King Juggernaut Drive》. And with the help of everyone, I defeated Trihexia.

I sealed then Trihexia's power in the Dimensional Gap guarded by Great Red and sealed his soul in a Medallion like how the Biblical God sealed Ddraig in the 《Boosted Gear》. The Medallion was sealed extensively six hundred and sixty-six times and was always kept around me to prevent others from trying to free Trihexia and thus ending the war.

In light of our outstanding performance and accomplishments, Sairaorg and I was promoted to Beelzebub and Asmodaeus respectively while Ajuka Beelzebub-sama, like Azazel, retired to be a full-time researcher. Vali was nominated but he refused as he found the position too 'restricting', but the real reason was because he was dying slowly (I only found out when he challenged me).

After taking the mantle of Asmodaeus, my perversion lessened (I just hid it better) and took the responsibility seriously with my lovely assistant, Ravel, of course.

I started my peerage with Ravel and Le fay-chan as bishops. I also requested a trade with one of my Knight pieces to Souna-senpai for Bennia to which she accepted.

Then 2 years after the war, Vali eventually caved in to Kuroka's advances. They had a child, a strong one like Kuroka wanted, but they never married. Bikou and I were named the Godfather of their daughter.

I was shocked when 6 years later I was approached by Vali, who revealed that he was slowly dying due to Trihexia's curse and no magic can heal him. He asked me for one last, final fight to determine who is stronger.

Vali and I fought in the dimensional gap with Ophis and Great red watching the fight of the strongest Sekiryuutei and Hakuryukou. We traded heavy blows, fatal hits, we were both relentless. In the end I won but not without suffering heavy injuries. Vali died as a result, he was killed with 《Ascalon》 by my hand. I apologized as I told Kuroka what happened and she was not surprised, she said that she knew Vali was dying and he would want to die by his rivals hand.

14 years after the Khaos war, Underworld and Heaven was now repaired with the aid of various factions. I also confessed about me being a devil to my parents who, to my relief, accepted my nonetheless.

Souna-senpai's school was now fully repaired. Kiba, who was crippled by Trihexia's Curse, served as a swordsmanship instructor and has now 2 apprentices. Koneko-chan reconciled with her sister, Kuroka. The angel population rose quickly after the war due to the 《Brave Saint System》and participated in rating games against Devil peerages which improved relationship between them.

Prologue End

Author's Note: This is my first story so please review. recommendations are welcome and no flames please.