Hello everyone! It's Gunmare back from the dead to bring out this new crossover story of High School DxD and Fate/Stay Night called Fate/Infinite Zero. By the title alone, hopefully you get the idea that the setting takes place during the 4th Holy Grail War where Issei Hyoudou is competing instead of Lancelot, which results in a powerful butterfly effect not only to the timeline but to the rest of the participants, masters, and servants in general if you get the chance to read. That being said, half of the servants participating in the Holy Grail War will have a different class from the original series, so here's your warning.

Also, anyone wondering what Issei's power level coming into the Holy Grail War, his power level is nearly identical or even stronger than his current canon version in the Light Novels (which up to the date this publish is Shin High School DxD vol 2) and this takes place after the Azazel Cup if anyone is wondering.

Please follow, favorite, and review if you like the story. Any questions please don't be afraid of messaging me or leaving it in the review. Also there will an extra Author's note at the end of the Prologue.

"Kill them all!"

Those were the last words he needed to hear from his contractor. The wyvern he sent with a leaflet managed to arrive at the warehouse undetected by the other masters and heroic spirits. It soared high above everyone, releasing the small piece of paper into the air before dissipating away into small particles of light.

"What the...?" Waver Velvet was the first to notice the leaflet coming down the ground before anyone else did. Rider picked up his master's gaze on the odd flyer and hummed, catching the attention of all the other participants of the Holy Grail as they glue their eyes on the leaflet. It safely landed on the four-lane road near the street light Gilgamesh stood on.

The magical crest on the leaflet lit a red glow as it formed a magic circle. It expanded beyond the border of the paper as it set itself ablaze. A pillar of fire erupted, its blinding, strong flames lit up the cold night sky to warm morn for only for a moment. An immense surge of power followed through, causing the ground to tremble, and the air and gravity too dense for the masters and even the servants to move and breathe.

It had to be the last servant, Berserker. There was no doubt in everyone's mind as the burning pillar died out, the ground settled, and the density of the atmosphere had alleviated itself to become easier to budge and respire. All the competitors held their guard, unsure what's to come from the mad-enhanced heroic spirit. Imagine their bewilderment once they saw a typical high school boy stood upon the same place where the spectacular performance happened.

"Yo!" The high schooler said, doing a two finger salute.

"He's Berserker!?" Diarmuid was the first to speak out, breaking the long awkward silence that came from seeing the strange young man appeared before them.

"But he's nothing more than just a lad," a servant dressed in a 19th-century French general outfit remarked, stumped.

"Yeah, he looks so ordinary and unimpressive like the boy," the Rider servant, Iskander, noted, referring to Waver.

"Hey! I resent that!" Waver retorted.

"Well sorry for looking a bit underwhelming!" The mysterious young man apologized with a slight blush on his face. "Despite looking like, in truth, I too bare the title of king and Emperor."

Brown hair dark as chocolate and brown eyes bright and vibrant as honey, and his facial features made more of Asian descent. This man was no descendant of her, Artoria was certain. Still, the King of Knights can't help to feel there a strong connection she and the man shared, leaving her silent. His element, she realized. The red dragon element resided in him like her.

"You a king?" Gilgamesh's cold glare sharpened. Attribute: Star. Alignment: Chaotic Good. Traits: Demonic and Divine, King, and Dragon. Those were the facets Gilgamesh picked up from looking at the man that he can easily tell was no heroic spirit of this world or even this omniverse.

So the aberration of a mongrel had the audacity to show himself. The reason why half of the servants included Gilgamesh himself materialized themselves in the wrong classes, why there's a heroic spirit that shouldn't be here, and why the entire Holy Grail War and even the timeline is out of whack was because of this foreign mongrel that dared to show up to the King of Heroes's domain.

~Ise's POV~

I felt the presence of eleven individuals here. Five, no six came from the Heroic Spirits, but that should be impossible. I'm for certain Assassin was taken out by the blonde-haired man standing high and mighty on the street pole, but I can detect another being similar to servants nearby.

Besides the servants, I discerned the presence of four other humans. There was one in the chariot belonging to the large redhead man I assumed to the Rider Servant. Or is it the blonde hair woman riding the white stallion horse? No, she's holding a lance, maybe she's the Lancer. That's beside the point. The other three were spread out in the shipping docks, though I noticed two of them were close by one another. Are they in cahoots?

Lastly was the beautiful red eyes white-haired woman dressed in the white winter's outfit. She was neither human or a Heroic Spirit. What was she? I'll probably use my Pailingual on her later.

'The caster... Kill him, Issei. Pulverize that Caster!'

I mentally sighed from my contractor's telepathic words. He sounded jittery and hectic. If he doesn't calm down, then surely those worms inside of will feed further into his deteriorating brain and internal organs. I can't let him get any worse than he already is.

"You a king?" I raised my head in the direction the cold, questioning voice came. My night-visioned gaze locked eyes to the blonde-haired man, my target, on the street pole. There he stood, scowling fiercely in an open short navy-blue vest with red and gold trims, exposing his bare chest. It matched the regal red pants that were embellished in gold jewelry. He had a golden gauntlet on his right hand, which held a stone tablet. Lastly, he wore a turban-like headpiece, which had blue-accented ornaments and veil that reached down his upper back.

"Tell me, Mongrel. What kind of a king is an ignoble imbecile like yourself of?"

"Glad you asked," I grinned, raising my left hand. As I point my finger to the night sky, the Boosted Gear materialized and my other hand akimbo. "I'm the almighty Harem King!" I announced loudly to everyone to hear.

"Harem...?"

"...King?"

Nearly everyone face faulted from my title as I sadly suspected it to be. Even blondie almost fell off the street pole. The smile on my lips from the joy announcing my kingship went bitter from my opponents' reaction.

"Um, excuse me?" The redhead man got my attention after letting out an uncomfortable cough. I believed his name was Iskander the Great. "Is that it?"

"What do you mean?" I asked back.

"It's your title," said a man holding a large black cannon with golden accents. He's Napoleon if I remember correctly from earlier. "Frankly I see no merit in your kingship."

"Eh!? Is there something wrong with being a Harem King?"

"Well, no." The redhead man sighed. His face frowned as if he was unsure of how to approach the matter. "But if you claimed a title as a king as we do, it symbolized the ideals and accomplishments of your kingship. By claiming to be a Harem King, you're telling us your greatest feat as king was developing a harem and only that."

"Oh, that kind of made sense, I guess," I said, thinking out loud. "Then if it's more suitable for everyone then, I shall use my other title then-"

"Enough!" A golden light shined down on me while I felt a heartless gaze filled with malice directed at me. Turning back up at the street pole, the golden-haired servant, my target, had four golden magic circles I had never seen before, reminding me of rippling water. Realizing it too late, I became tethered down by magical chains of golden light. I bit my lip, restraining myself from screaming at the pain of dense power of light and divinity of the chain sinking into my body, which incinerated parts of my clothes and burned on my flesh.

"How dare you, mongrel..." Caster barked, grinning his teeth. "Not only you trespass to my domain like the invasive parasite you are, but you dare make a mockery of the title of king in front of me! May your death provide me a modicum of entertainment."

As golden beams of light fired down at me from the wands, my last thoughts were, Ah shit.

Artoria's voice had yet returned. She observed the events unfold, conflicted. There were many heroes from legends that bear the trait of the dragon within them, but none match the criteria Berserker was. Her emerald eyes were mesmerized by the gauntlet he summoned on his left hand once Berserker claimed the title of his kingship, even after almost falling her beloved horse, Dun Stallion.

"Lancer," Irisveil's soft concern voice awoke Artoria from her daze. The King of Knights looked down at her white-haired friend as she leaned close to toward Lancer and her steed.

That's right, Artoria gave Irisveil an assuring smile, comforting her that she had everything under control. Knowing Berserker's identity wasn't the issue. Surviving tonight's affray was. If the two servants managed to see tomorrow, Artoria would love to know more about Berserker and his connection the red dragon if their paths were to cross.

"Not only you trespass to my domain like the invasive parasite you are, but you dare make a mockery of the title of king in front of me! May your death provide me a modicum of entertainment."

Artoria swiftly looked back at Berserker's direction right as Gilgamesh fired his noble phantasm down at his confided pray. A flash of golden prana showered down on the concrete road, disintegrating everything into smithereens. Everyone shielded their eyes from either the blinding light or the thick clouds of smoke caused by the explosion that gushed through them. Berserker was chained down and defenseless when Gilgamesh launched his attack. No one in his position could survive such attack, but Artoria knew he did. Before the blast struck him down, Artoria heard something coming out from his gauntlet. Those words were Welsh Dragon Balance Breaker.

The entire warehouse became buried in a field of smoke. Among the dark clouds, multiple small green lights pierced through the destiny of the smog, becoming the beacon for everyone's eyes and an indicator of Berserker's survival. Another gush of wind blew through the warehouse, sweeping the smoke away, giving everyone a clear view of what has happened to Berserker. The Harem King continued to stand tall unscathed and no longer bound by Gilgamesh's golden magical chains. His body donned a set of red medieval armor with multiple emerald orbs oriented on his arms and legs and one large one on his chest. His face hidden underneath a red helmet that resembled the shape of a dragon.

I managed to activate my Balance Breaker in the nick of time before the blast of prana had a chance to struck me down. I broke through the chains once my armor materialized and without prioritizing the next course of action thoroughly, I quickly countered back by firing a barrage of dragon shots. There was no time to either navigate them or increase their strength and durability with the Boosted Gear since Caster's attack already was closing in on me. Some of it managed to break through space between the multiple dragon shots and not get itself caught in the explosion once the two attacks overlapped one another. Luckily though, it went around me and struck the road instead.

Spreading my dragon wings, I blew the smoke away with a single flap. They were quickly retracted back to my armor after clearing the field. I watched the expressions my opponents had once I activated my Balance Breaker. It was a mixture of awe, confusion, excited, stoic, and terror. Caster remained unhinged, no rather more annoyed as his dagger eyes pierced through my soul.

[That was a close one, Partner,] Ddraig said inside my mind. [If the one observing everyone here is Assassin, you would have been the first participant out.]

"Tell me about it," I said under my breath. "I'm facing off the real heroes from epics and myths, and they're nothing like the ones I face in my world."

[Obiviously. But don't forget, we're in a world not of our own.] Ddraig reminded me. [It doesn't matter if these so-called heroes shared the same origin and namesake as the heroes of our worlds. They possess different power and abilities none of the heroes in our world share; however, I might have figured out the blonde magician's real identity.]

"Really?" I blinked in surprise.

[It's just a hunch,] Ddraig admitted. [But I was able to analyze his mana from his chains he restrained us in. This man is a demi-human, a descendant of man and god, but his biology is more god than man. Judging by this and his attire, his real identity is...]

The mongrel survived. Gilgamesh clicked his tongue in contempt.

Berserker managed to break free and evade his attack faster than the King of Heroes predicted. His holier than thou eyes witnessed everything from Berserker's transformation and breaking his magical chains in a barbaric manner to retaliating against Gilgamesh assault with one of his one. Gilgamesh readied himself to draw out more of his wands out to finish Berserker off but stopped as Berserker spoke out for everyone to hear.

"You almost had me there, oh great king of Uruk, Gilgamesh." The King of Heroes' eyes widened only for a second, realizing the outsider figured out his identity instantly. The burning odium he harbored for Berserker flared up, frustrated that the outsider discern his identity first before any of the lesser beings and fake kings of his world did.

"Did he call him Gilgamesh!?" Waver spoke out from the carriage, his mouth gaping from the revelation. The other masters and some of the shared the same reaction as the Master of Rider. A trickle of sweat ran down on Kiritsugu's head, his mind racing as he reprioritized his strategy. Kayneth slammed his fist into one of the containers, cursing profanities at the Tohsaka household for summoning a Heroic spirit far superior to his own. Tokiomi, back at his mansion, rose from his stool, distraught. Anxiety's cold touch slithered down his spine as foreboding and uncertainty clouded his mind.

The probability of the other masters and heroic spirits knowing Gilgamesh's identity was low. Even if they did discover it, Tokiomi assumed it is only late in the game; however, Berserker managed to figure it out in a blink of an eye. Soon the other masters will come after him for being the biggest threat to either eliminate Gilgamesh from the war or recruit him once they had disposed of Tokiomi.

"Wait, Berserker," Tokiomi muttered, sensing something was off. The manner Kirei described was rather peculiar as he arrived through the use of a magical flyer rather than materializing from his spiritual form and there was also the issue he hadn't gone insane by the Mad Enhancement. While there had been cases where a Berserker servant won't lose their insanity from having Mad Enhancement, Tokiomi knew in his heart and mind that Berserker doesn't possess such skill.

Then Berserker's master crossed his mind. With everyone being at the warehouse, it wasn't hard to figure which partnership between the masters and heroic spirits. He and Kirei respectively had the Hassan the Hundred Personas and Gilgamesh as their servants, representing the Assassin and Caster in the war. The horse riding Lancer belong to the Einzbern, Kayneth had the green-clad Saber fighting for his corner, the Master of Rider stood in the chariot with his master, and the Archer servant arrived after the three Founding Families summoned their own.

"Oh, Kariya," Tokiomi sat back down on his stool and put his hand on his throbbing forehead. "What in God's name have you done?"

Kariya Matou was the former heir to the Matou family and good friend to Tokiomi's wife, Aoi. According to his wife and the current head of the Matous, Zouken, Kariya was the only one in his family to be born with magical circuits but gave up a life of being a mage and walked out on his declining family, leaving no proper heir to take over the family. He returned just recently to participate in the war after learning that the Matou had adopted Tokiomi's daughter into their household.

Tokiomi had Assassin to keep an eye on him and his servant on the night he summoned him, but the Hassan arrived at the sight of a burning mansion. There was no bounded field around the area when Assassin entered the land as if someone destroyed it. According to the reports, there were no inhabitants at the fire occurred as Assassin found no traces or remains of a single body after the fire was put out. Hearing it gave Tokiomi some peace mind, knowing his other daughter is still alive, but worried about her whereabouts, though he hadn't sent Assassin to look for her or told Aoi, who still unaware of the incident that occurred."The situation is turning for the worse," Kirei's voice said coming from the record player in Tokiomi's basement. "Gilgamesh is getting riled up. If this continues, Gilgamesh might open the Gate of Babylon even further and wipe everyone there. What are your orders, Master?"

Tokiomi stared down his command spells, conflicted on what to do. If Berserker hadn't blabbed out Caster's name, Tokiomi spent no time using a command spell to reclaim his Servant before things escalate any worse in their favor. He probably will need to use one, but not just yet. With all the partakers of the Holy Grail War gathered in one place, Tokiomi distinguished everyone's identity. All but one.

Berserker's identity remained a mystery. His appearance, noble phantasm, or his king's title gave no hints to his real name. Tokiomi won't let his servant's identity go out in vain. If Gilgamesh can get Berserker to reveal his real name soon, then tonight won't be a total loss.

"Judging by your reaction, I reckoned I hit the bullseye," I concluded, retracting his helmet to show his smiling face. "What's wrong Gilgamesh? Cat caught your tongue."

"Hold thou tongue, Interloper," Gilgamesh hissed back. More of those magical golden portals from before appeared around the golden-haired mage, each one armed with their own unique golden wand. "How dare you sully my name with that foul mouth of yours. I offered you a swift and painless death out of my world, but I see I you desire to perish in the slowest horrendous way possible."

"Sorry blondie," I said, having my helmet back on. "That's not going to happen. I have a contract to fulfill and a wish to claim!"

[Boost! Boost! Boost! Boost! Boost!]

All the green gems on my body lit up as my power level increased by thrity two times than normal. My increased power level didn't go undetected by the others from the bewildered gazes that fell upon me. I paid no attention as Gilgamesh conjured his wands to fire at once, this time launching a horde of magical bullets down at me. Mimicking him, I created multiple dragon shots in both my hands and lashed them all out in two separate waves.

I maneuvered the first wave to intercept the bullets. Their head-on collision exploded like fireworks; Gold and red particles scattered through the air in the cloud of smoke containing them. Using the smoke for a cover, I sent out the second wave of dragon shots and directed them at Gilgamesh and his arsenal.

My opponent grimaced, fleeing off the top of the street light before my dragon shots struck it down along with his wands. Before his feet touched the ground, red flames were ignited from the thrusters on the back of my armor, leaving a streak of red behind as I dashed forward, ready my fist to deliver a finishing blow.

Due to being caught up by the momentum and closing on Gilgamesh, those golden magical appeared by his side. A weapon emerged from it, a golden ax. Gilgamesh grabbed hold of it and held it in front of him with the blade directed on me.

"Ascalon!" I cried out, the blade of my holy sword popped out from the gauntlet.

A jarring chill struck both of us once our blades clashed, gushing out sparks of holy aura from our weapons. The clash detonated a powerful shockwave, sending us skidding back across the road in opposite directions. Gilgamesh halted his body from ramming into the containers through the use of his ax. He slammed the end knob of his weapon into the ground to slow down his hurled body. Luckily, the weight of my armor held me down, and I wasn't flung so far compare to Gilgamesh.

"Fool..." The king of Uruk growled, his shoulders and eyebrows trembled from either rage, irritation, or the combination of the two. "I belong up in the heavens, yet you dare have me trod the earth as you!? You-"

"Oh no, Gilgamesh," I shot back, interrupting him to his vexation. "I'm here to do much more than that. I let you in on a secret. Just like you belong up in the heavens, I'm a devil from the underworld that will drag you in the depth of hell! In the name of my kingship and my position as the Sekiryuutei, I will come out on top! Promotion: Queen!"

I don't belong in this world. I shouldn't be partaking in this sacred competition between mages and legendary heroes of the past for a chance to have my wish granted, yet I'm here doing that. As the alignment of my evil pieces altered, the memory of the last five days flashed through my eyes. I can still see it vividly as the memory replayed in my mind. The memory of the day I decided to take a contract through the teleportation circle and not my bike.

Hello again everyone. I hope you enjoyed the Prologue in this story. Just to let you know I originally wanted to publish this back in January but couldn't due to me being in college and working at my part-time job.

As I stated the servants in the Holy Grail War had their classes changed due to Issei's summoning in the war instead of Lancelot. I'll give you a list of it right now and status on what's Issei's relationship between the other masters and servant in the war but only to some of them since I don't want to spoil too much of the story. I'll try either making a chapter with all the participants or include one in every chapter. Or credit it in the prologue when I get the opportunity.

Saber: Diarmuid Ua Duibhne

Master of Saber: Kayneth El-Melloi Archibald

Archer: Napoleon Bonaparte

Master of Archer: Unknown (For now)

Lancer: Artoria Pendragon

Master of Lancer: Kiritsugu Emiya

Rider: Iskander

Master of Rider: Waver Velvet

Caster: Gilgamesh

Master of Caster: Tokiomi Tohsaka.

Assassin: Hassan of Hundred Faces

Master of Assassin: Kirei Kotomine

Berserker (?): Issei Hyoudou

Master of Berserker: Kariya Matou

Also, anyone in the audience out there that want to contribute to helping me with this story, I do need a proofreader/Beta Reader to look out for any mistakes or errors in my grammar or characterization in the characters I still feel a bit uncertain of some of the characters. Okay time for the preview of the entire story

Ddraig: Partner, I think we're no longer in Kansas in Anymore

Issei: Kansas? Ddraig, I'm talking about Kuoh Town here!

Before the confrontation between the other master and servants, Issei must come to the realization that he is a different world of a different time.

Kariya: This can't be happening. I thought the summoning was going to botch

The summoning of a complete stranger put the already stressed out Kariya in a dire situation as he is unaware he just hit the jackpot.

Diarmuid: In the name of my master and my honor of a knight of Fianna, I will slay for interrupting my match against Lancer and for almost killing my master.

The relationship with the other masters and servants are coming off a rocky start.

Iskander: So there are other universes out there, eh? Interesting...

The discovery of other worlds outside of his own had piqued the interest of the King of Conquers. His thirst for conquest show no bounds.

Napoleon: Sacre bleu! I can't believe the great King of Conquers had invited me to a banquet.

Another servant that was never meant to appear had shown up. What special role does his unknown master want him to play in this war?

Gilgamesh: The reason you can't figure out the Interloper's real identity is that he's not from our world or even this omniverse. He is who he said he is. He's a pesty devil that must face divine punishment by my hands. Maybe this is the reason why I was summoned as a Caster in the first place.

Summoned as a Caster instead of an Archer, Gilgamesh's personality is altered. He isn't the arrogant and egotistical king that would treat Issei nothing more than an eyesore, but as the wise-king realizing Issei is a threat but not only to the Holy Grail War but to the whole world.

Artoria: Tell me, Issei. A fellow bearer of the Red Dragon. What does the title of the king mean to you?

In inventable Banquet of Kings, Issei will share out his opinion on what it means to be a King and reveal his wish for the Holy Grail War.

Sakura: Will you go after the war is over?

Issei: I'll always be there for you, Sakura-chan. Even if I were to disappear one day, part of me will forever stay in you. Both in your heart and in your dreams.

Next time in Fate/Infinite Zero: A Pact with the Devil

Issei: I'll do more than fight. I'll win for your sake, Sakura-chan's sake, and for myself.