Disclaimer: I don't own Harry Potter or One Punch Man.
'TROLL IN THE DUNGEONS!' That was what their Defense against the Dark Arts professor had literally shouted out the Great Hall of Hogwarts School of Witchcraft and Wizardry during their annual Halloween feast. After his over the top exclamation, Professor Quirrel, their aforementioned professor, who was widely regarded as a coward who stuttered almost each and every sentence that came out of his mouth and it was assumed he wore a garlic perfume, as weird as that sounded, fainted with a loud thump that resounded within the Great Hall, seeing as his announcement had caused all of the conversation within to stop.
The screaming shouted immediately, followed by all out chaos from the sides of the students of Hogwarts. It wasn't every day when you got a troll inside your school every day, which was also meant to be the most secure location in all of Magical England. Combine that with the hyperactive imagination of the young wizards and witches that were present, it wasn't surprising that everyone immediately freaked out. Although 2 wizards were freaking out for a totally different matter altogether.
Ronald Weasley was one of them because he suddenly remembered that he was slightly rude to a witch earlier, namely Hermione Granger, just because she tried to help him with the Levitating Spell. He thought that she was being totally obnoxious even though she was simply trying to help, and with him trying to nurse his wounded pride at seeing a muggle born besting him in a practical field of magic. He had called her a bookish, obnoxious know-it-all who had no friends while exiting their charms classroom, which incidentally she had and caused her to flee the site in tears. She had not been seen since, even skipping the Halloween meal altogether. Ron finally noticed that she was not present after Headmaster Dumbledore ordered the houses to go back to their dormitories immediately.
He quickly asked around, so as to find her and apologize. He wasn't a bad guy after all, and he knew when he was wrong and to apologize.
Neville Longbottom was the other one, and he was freaking out for another matter entirely. Neville was the considered by the wizarding world as the Boy-Who-Lived, for he had brought about the downfall of the dark lord, Voldemort when he was a baby, exactly 10 years ago. Nobody knew what exactly had happened on that fateful day, but the general idea was that the Potters and Longbottoms, both Ancient pureblooded families, widely regarded in the wizarding world, had gone into join hiding from a year for some unknown reason. Frank and Alice Longbottom, the parents of Neville had left their joint residence to pick up supplies, while James and Lily Potter, the parents of Harry Potter had stayed behind to protect Harry and Neville. There was an information leak of sorts, and their hideout was discovered. Instead of sending a usual assassination squad, Voldemort had seen it fit to dispose of his enemies himself. What followed was James and Lily Potter's futile attempts to delay the dark lord in order to protect the year old boys. When they failed, it was mostly assumed that Neville invoked some powerful ancient magic to shield both of them, causing the only Unblockable curse, the Killing Curse, which was a personal favorite of Voldemort's repertoire to backfire on himself. The Dark Lord was dead by unknown means, and the magical world rejoiced. However, their relief was not to last long, as the Longbottoms were attacked immediately after a few months of coming out of hiding, surprisingly due to another information leak, only that they were attacked by followers of Voldemort, seeking revenge. The Longbottoms were placed under the torture curse for an extended amount of time, causing them to lose their sanity. The offenders were caught and were immediately given the worst punishment ever, but the damage had been done. Neville was placed under the care of his grandmother, who on Dumbledore's advice started turning young Neville into a young soldier so the mistakes of the past could never be repeated. She wanted to make sure that Neville would be safe, and would have to strength to ensure his survival and the survival of the ones that were close to him.
Neville remembered those days fondly. There was not a single day where his tutors would go soft on him. They kept on imparting all kinds of magical theory and the art of fighting to him in a very painful way. Failure was barely tolerated, success was met with encouragements to do better next time. They kept on pushing him to overcome his boundaries, making Neville believe that there was no target out of his reach, as long as gave it his all. A few years earlier, Dumbledore had visited him and had confided in him that the fact that Voldemort was not actually dead, and he believed that Voldemort would make his move to attack him while he attended Hogwarts. That had led to his training in combat magic accelerated, and as of the date, he was capable of dueling a 7th-year student to a draw. Also, Ron did mention that Hermoine was not present, all thanks to the fiasco that had happened after Charms. He now had a person he wanted to save, and he set out to do it.
Hermoine screamed as she dodged another badly aimed swipe by the troll. The behemoth towered over her, but its aim with its club was abysmal. That was her only saving grace. She had just left the ladies room, just to crash into its leg. A quick roll to the side had saved her, seeing as its wooden club smashed into the place she was just standing. She dived into the closest room, closing the door behind her, hoping that it was enough. It was the men's room surprisingly, which gave her the option of hiding in the stalls. She ran into one of the last ones, hoping to the Gods above that the troll would go away. Unfortunately, at that moment the door was blasted inwards, and the troll lumbered in. Angered by the lack of its prey, he decided to take out his anger on the closest item, which was a marble sink mounted on the wall. That led to her sudden shrieking which further led to the troll smashing the stalls in an attempt to find her, only for her to hide in the wreckage, silently hoping for someone to come and help her.
Someone did.
Neville skidded into the room, his eyes widening at the carnage and at the troll. He flicked his wrist, causing his wand which was made of cherry, had a core of unicorn hair and measured at 13 inches to slide into his wrist from his wrist-wand holster. He aimed at the troll's midsection, and fired off a near silent stunning spell, only for the red stunner to fizzle out just as it hit the troll. He didn't have time to blink in surprise, as he had to backflip to avoid the clumsy attempt of a punch by the troll.
'Normal spells won't work Neville, trolls have hides which are moderately resistant to spellfire!' Hermione's voice shouted from the wreckage of the stalls.
He knew he made the right decision to come here.
His initial surprise fading, Neville slipped into his combat mode, firing a Diffindo- the severing charm towards the knees of the troll, trying to hamstring it. It didn't fizz out completely, leaving only the barest of red marks on the right knee. However, the troll didn't react to the minor paper cut at all, which led to Neville forced more power into his next Diffindo, aimed at the same area.
This time, there was a visible injury and an angry reaction from the troll. Which led to the troll giving a shout of rage, and increasing the speed of its swings. It noticed the debris on the floor due to the destruction of the stalls, and it half dragged and swung its club through the debris, causing it to go flying at great speeds.
Neville in return shouted out 'Protego' and a near translucent shield sprang up around him, protecting from the flying debris. The troll, however, did not simply stop there. It had a new target and it half bum rushed it, roaring loudly in the process. Hermione chose this moment to scramble out of the wreckage tried to immediately help Neville by casting 'Rictusempra', which unsurprisingly did have any effects. Neville cast a 'Reducto' which only caused the troll to be displaced by a few meters at the most and did not even injure it in the slightest. It did, however, give him some breathing room.
He noticed that Hermoine was attempting another spell. He shouted at her,' Don't do that! Just get out of here!'
'There is no way I'm leaving you here alone Longbottom!' She shouted back.
Neville opened his mouth to go and get a teacher for help was cut short by him having to dodge another overhead swing of the troll's club. Neville executed a perfect combat roll to his left this time, skillfully avoiding the debris and dodging the wooden instrument of death. He leaped to his feet and only had time to acknowledge a warning before he felt the troll's fist punch him with full force into the marble wall.
It hurt. A lot. Neville could only groan in agony as the fist let him loose, making him fall to the floor in a heap. He was sure that if he hadn't reinforced his body with runes before coming to Hogwarts, he would have been suffering from more than just cracked ribs. A scream brought some semblance of senses back to him, he barely noticed that Hermoine was at his sides and was attempting to drag him away from the troll, who simply was standing there, grinning like a loon.
That was when Harry Potter entered the washroom.
Neville barely recognized him, even if he had enhanced vision (again due to branded runes) but he was suffering from a huge concussion at that moment and was having a minor case of double vision. Harry's face was looking as if he was slightly constipated as if he was in a rush to go to the loo. Apart from that, he looked like the average, spectacled 11-year-old kid. Neville felt a stab of regret for never having to got to know Harry, seeing they both had survived the brutal events at the Halloween, 10 years earlier. Whereas Neville got to live with his grandmother, Harry had simply disappeared off the face of the earth, and only reappeared to enroll for his first year at Hogwarts. After enrolling, Harry then proceeded to be utterly proficient in practical magic, and completely useless in its would have laughed at the dark comedy, that they survived whatever Voldemort did, only to be killed by a dumb troll. Since laughing would have aggravated his chest injury, he settled for snorting.
He barely made out Hermione yelling at Harry to run, only for him to stare dumbly at her. And the troll. Which was looking at the newcomer with another dumb look on its face. Harry gave it a dumb look in return.
They dumb looks on the faces remained for 10 seconds till Harry's nearly full bladder reminded him again of his visit.
'Excuse me troll, I just want to go to the washroom for a bit, we can continue our staring contest later.'
That dialogue caused the troll to break out of its reverie, and it roared in Harry's face while bringing its club down for another overhead smash.
The club hit the ground with a loud thud.
Then the troll noticed that Harry was standing beside the club, glaring slightly at him. The miss angered the troll and it went into a frenzy, swinging its club as fast as it could.
Meanwhile, Hermoine and Neville were watching the event unfold with wide jaws, Harry was dodging the blows with inhuman speed. He wasn't simply dodging the offending piece of wood, it was as if he was just apparating into another spot. There was no roll, no flashy acrobatics. He would be at a spot, the club would hit that spot and then Harry would just appear somewhere else with a just a minor dust cloud to signify that he had moved at all.
'How is he apparating?' Hermione asked out loud 'I can't even see him twisting to begin the apparition process! Also, you cannot apparate on Hogwarts!'
'That's because he's not apparating, he is simply sidestepping the blows. With extreme speed.' There was pure awe in Neville's voice. The speed of moment Harry was using, he had never seen it before, not even at the World Duelling championships by the winners.
Finally, the troll seemed to have enough of swinging around and it decided to take a pause. The troll lifted its club, expecting a red paste on its underside, only to find the surface more chipped than before. It narrowed its eyes in confusion, then looked down below, where Harry stood, frowning.
Then Harry moved.
Neville wasn't sure Hermione saw it. Harry was moving too fast for her to keep track before, and he was sure that she didn't see him pushing against the floor so hard that it cracked. She didn't see Harry jump in the air and point his wand at the troll's head. To her, he just appeared in the air and shouted 'Reducto' which caused the troll's head to explode like a ripe melon under pressure. Neville did see what Harry exactly did, thanks to his enhanced vision. He saw Harry reaching the peak of his jump exactly at the troll's head. He saw him point his wand and blast off a powerful 'Reducto', which so overpowered that it tore the troll's head clean off its body. After which it exploded.
Harry had killed the troll with just one spell and pure speed.
The body of the headless troll fell to the floor with a loud thump, and Harry landed beside it as if nothing was wrong in the world. His annoyed expression turned back into his dumb, constipated one, and he rushed into a semi-ruined stall, ignoring the gobsmacked expressions of his two classmates.
It that scene to which the teachers arrived, two utterly bewildered students ( one injured ), an almost demolished toilet, a headless troll, and sounds of relief coming from a half-ruined stall.
Author's note: This idea of a plot was annoying me since like forever. Finally, I decide to write it down, to give myself a full night's sleep. This is technically not a One punch man crossover, no characters from the OPM universe will be dropping in the HP universe. It's just that some HP characters will gain the characteristics of some heroes/villains from the OPM series. Most importantly, Neville is Genos and Harry is Saitama. I guess that should be obvious.
This is only a chapter based fight only story and all of the chapters will be focused on the major fights that Harry will and would have normally book-wise. There will be no bashing and only minor exposition on the world's events before or during the fights which will tell you the events up to date.
I am not portraying Dumbledore as evil, he has just wrongly assumed that Neville was the BWL. As for why Neville got trained and the rune brands, he cannot be Genos without training and some semblance to the cyborg, right? ;)
Also, I'm really look out for a beta who is willing to help me with my abysmal grammar.