HPMWE 1

Authors Note: There will be some differences from the movies; it being a crossover is one reason, but Psylocke/Elizabeth Braddock will be British, like in the comics. Harry was born August 18th, 1978, two years before canon and is a mutant.
I'd like to thank the transcript wiki, marvel wiki, and X-men movies wiki. You provide an excellent source of information and invaluable resources for fanfiction writers.
Now, onto the story!
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Harry Potter's life sucked. Badly. His parents abandoned him after he accidentally depleted his magic while being subjected to the Cruciatus Curse by Lord Voldemort and after trying to defend his baby sister... they thought he was left a squib because Dumbledore said so. Harry was left in an abusive environment with his Aunt Petunia and her revolting husband... Harry was forced to cook, clean, and sleep in a cupboard.
This continued until Harry discovered that he was a mutant and then forced them to treat him like an actual human being. You see, Harry discovered that he could mimic the powers of other mutants, and knowledge and skill, and he was fortunate enough to acquire several powerful abilities; Telepathy and Telekinesis among them. That and his magic, which despite Dumbledore's faulty assumption still remained, had allowed Harry to draw on his magic to augment his mutant gifts and give them permanently increased power and range. Not only was Harry a powerful mutant, but he had grown his power to that of a Mage. Destroying the Horcrux within his forehead gave him the magical knowledge that he needed and Harry could always mimic more.
Harry's life somehow got worse when a blue guy killed his aunt, uncle, and cousin and abducted him. Mere minutes ago. It was also all Harry's fault. He was training his Telepathy and it was traced back to him. Harry was lucky that he ended it before he had to combat mind control.
"Hey, you big blue idiot, you do realize that I can easily kill you, right?" Harry asked boredly, causing Psylocke and Magneto to both briefly smirk at the insult.
"I doubt that very much, child." Apocalypse said condescendingly.
"Oh? An atmokinetic, a telekinetic that forms her energy into weapons, a guy with metal wings, and Magneto himself. And then there is you: perhaps the second strongest mutant alive, a master telekinetic who's telekinesis borders on matter manipulation, powerful albeit limited telepathy, near instant regeneration, immense strength and durability...." Harry said, shocking Apocalypse and his horsemen. "Did I mention that I can sense and also mimic powers at their full potential? Oh and similar abilities have their power and skills added too on top of pre-existing abilities."
They all looked extremely nervous now, even Apocalypse.
"Now, I have no intention of squashing you idiots like the annoying insects that you all are, but put me down before I change my mind and decide to torture you all until you all beg for the sweet release of hell's eternal torment. I can only tolerate so much annoyance and carrying me around like a toddler is a sure-fire way to push me to my limits." Harry said and Magneto calmly put him down, seeing some of his younger and much angrier self in Harry. "Good. Now that I don't have the urge to burn you all alive, I'll stick around and watch you idiots get your asses kicked and let the guy with a face that not even a mother could love try possessing my body as his new vessel." Harry said, to Apocalypse' shock.
"Oh? Was that a secret? Don't worry, you'll fail and I'll come out of this stronger than ever." Harry said, but Apocalypse didn't believe him. "That's fine, don't believe me. Just keep exposing me to more mutant powers until I outclass you even further. That'll be loads of fun."
"I won't be letting you gain any more power, not until I can take your body for myself." Apocalypse said dismissively.
"There are literally dozens of dirty jokes I can make about that." Harry deadpanned and had to fight the urge.
"Aren't you a little young to be this well-spoken and... mature?" Magneto asked cautiously.
"I mimic knowledge and skills too. And I made the mistake of age shifting into a teenager to mess with some people, and that messed with my hormones. This has left me with the equivalent of several degrees and the urge to make pedophile jokes to anger that creepy old dude." Harry replied.
"You really shouldn't do that," Magneto replied, disapproving of Harry's foul mouth.
"Old guys shouldn't want to be inside little boys." Harry replied, causing all the horsemen to laugh. Sadly, all of Harry's next attempts to amuse himself were firmly halted by Apocalypse and Harry decided to just flirt with Storm and Psylocke, who just looked amused and flattered by it, and pretending to believe that Angel was a butchy female. Harry could tell that it was just his age that was hindering him from getting Storm and Psylocke from dating him, but he wouldn't be five forever....
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Harry was bored out of his mind and that delusional blue arsehole with a God-complex was preparing his 'horsemen' for battle. Harry was genuinely concerned that he might have to battle the blue bastard himself and that was very scary for him. Harry didn't like his odds of victory with that much power being brought to bear against him, and him mimicking abilities only did so much. Harry had a plan, of course, but it would require Apcalypse actually trying to possess him.
Harry noticed Magneto leaving the other evil minions and his master and looked into his mind... and found another telepathic presence.
"They would have wanted to live. I tried your way, Charles. I tried to be like them. Live like them. But it always ends the same way. They took everything away from me. Now... We'll take everything from them." Magneto said.
"You should withdraw, whoever you are. You're about to get traced and possible even taken control of." Harry said, speaking to the telepath through Magneto's link with him.
"Another telepath? How come I can't read your mind?"
"Psi-Shields and now it's too late. En Sabah Nur has discovered you." Harry said in annoyance.
"Extraordinary." Apocalypse said.
"What do you see?" Angel asked.
"The answer." Apocalypse replied mysteriously.
"Meaning the damn parasite found someone new to leach off of... yay." Harry said snarkily.
Thankfully, Apocalypse only got rid of the worlds nukes, but Harry tagged along for their trip to this Charles persons location.
Once they arrived, Harry mimicked the abilities of the guy in the wheelchair, the guy who could absorb energy, and the endurance, strength, and knowledge and skills of the tech support looking dude. With him gaining their abilities full potential, Harry had grown his power a great deal. But it was mimicking Mystique's mutant ability that Harry liked most, he got himself a much better shape-shifting ability. Over all, Harry was quite pleased with this shopping trip. Magneto pulled Charles to him and then Apocalypse opened a portal and Apocalypse, the Horsemen, and Harry and Charles all left.
Hours later.... 
Harry was the first to notice that Charles Xavier was waking up and Xavier's pitiful attempt to invade his mind quickly irritated him.
"You're blocking me. How?" Charles asked.
Harry snorted in derision. "It's not exactly hard to do." Harry was honestly disappointed by Charles Xavier's true power and skill, he needed to use technology as crutch and couldn't even shield his own mind.
"I can shield their minds from your power. It's one of the many gifts I have acquired through out the millennia. But to... see... inside a mind. To control it. That's your and Hadrian's gift. You saw it, didn't you? The glory of what's to come." Apocalypse replied.
"Are you going to take part in all this killing? Destruction?" Charles asked, looking directly at Erik.
"It's all I ever known." Magneto replied.
"No, it isn't. You've just forgotten."
"No, Charles, I remember. Your way doesn't work."
"I showed him a better way. A better world."
"No, you just tapped into his rage and pain. That's all you've done. I told you from the moment I met you, there's more to you, Erik. There is good in you too." Charles replied. Personally, Harry felt that Magneto sounded much cooler as a name.
"Whatever it is you think you saw in me, Charles. I buried it. With my family." Magneto replied.
"You two are very dramatic for supposedly straight guys," Harry interjected causing Psylocke, Angel, and Storm to laugh. "Angel's supposedly straight too. You three should get along great!"
"Shut up, runt!" Angel said angrily while Storm laughed. Even Psylocke looked conflicted about not laughing with Storm.
"Take your own advice, fairy," Harry replied and Angel moved to attack Harry. Harry telekinetically flung him into a nearby rock formation and did it casually and with major style. Angel struggled to get up. "You just got your ass kicked by a five-year old!"
"Enough!" Apocalypse bellowed, halting Angel's next attack and the entire pointless battle.
"Charles, you will send a message to every living mind. You will tell them that this Earth will be laid waste. That it is I, En Sabah Nur, who wreak this upon them. Now. Deliver my message."
"Even if I wanted to, I couldn't reach that many people. I don't have Cerebro." Charles replied, causing Apocalypse to roll his eyes.
"You don't need a machine... to amplify your powers. You have me." Apocalypse replied.
"It's actually quite pathetic that you even need a machine to increase your reach." Harry added.
"Oh? And how far does your Telepathy reach?" Charles asked somewhat condescendingly.
"850 miles at present and I can increase that to over 1200 miles for two hours or permanently by mimicking the powers of other Telepaths and adding them onto my own." Charles was visibly torn between shock and disbelief. Harry could actually double his range for four hours, but nobody needed to know that. "Superior power and skill to literally everyone. Damn! I'm awesome!"
"Imagine what he can achieve at the height of his power.... That is why I'll be taking his body as my own." Apocalypse added, to Xavier's shock.
"Don't worry! He'll fail." Harry said dismissively. With that vague, seemingly arrogant, and completely unhelpful platitude, Charles began sending Apocalypse's dictated message with his own a hidden one within it. Harry just barely sensed it and was grudgingly impressed by Xavier for managing that. Sadly, Apocalypse wasn't amused when Charles went off script.
"This was meant to be the center of the universe before I was betrayed. Now. It will be." Apocalypse said, before beginning to telekinetically tear apart Cairo and build himself a new capital.
"You're an arrogant motherfucker, aren't you?" Harry asked boredly, causing everyone but Psylocke to look shocked. Psylocke looked mildly amused by a five-year old cursing like that. 
"Honestly, do you enjoy hearing yourself speak that bloody much?"
"Language!" Charles and Magneto said, despite mainly being focused on the destruction caused by Apocalypse.
"Yes, Mum and Dad," Harry responded mockingly. 
"Where are your parents?" Charles asked in concern and with Harry's mockery having brought it to mind.
"They abandoned me because they thought I lacked power like them," Harry replied and it wasn't untrue. "Blue boy killed my aunt and uncle, quickly disintegrated them and my cousin."
"I'm sorry," Charles replied, assuming Harry's parents were mutants who abandoned what they though to be a non-mutant.
"I'm not. I am more powerful than my parents could ever dream of, and there was no love lost between myself and my aunt, uncle, and cousin. I could have been kept by my parents and could have grown up neglected and possibly abused, but my aunt and uncle did that instead. I'm glad they showed me their true colors. It will make my revenge far more justified and amusing."
"Revenge won't bring you peace, Hadrian," Charles said.
"I know, but seeing them humiliated will be very amusing for me. I have no intentions of killing them though. Death is a release from the torment of life and I have no desire to spare them from any of that... I'll just be collecting some unpaid child support and collecting my inheritance with great interest."
"You could also move on and build a life of your own. You could probably even defeat Apocalypse right now if my suspicions are correct." Charles suggested.
"Your suspicions aren't correct. I can defeat him one-on-one, but he has numbers on his side with his horsemen and their power is nothing to sneeze at either. Sadly, my flirtation with Storm and Psylocke hasn't guaranteed their neutrality or aid, and I'm seriously willing to date them now by age-shifting or when I'm physically legal. Still. I would need another major power up-grade to safely combat them...."
"You won't have the chance," Apocalypse said, having just finished his renovation of Cairo.
"That's a matter of opinion, and yours is biased by arrogance and a God-complex." Harry replied. "Is it time for your attempt to possess me?"
"Yes, it is time," Apocalypse replied.
With so many people that could fly, getting to the chamber where Apocalypse would attempt to transfer his essence into Harry didn't take long at all. It was just Harry, Charles, Apocalypse, and, Angel, who would be watching over Charles and guarding Apocalypse in the chamber.
"Soon I shall be all-powerful! And your power and limitless potential will make it so!" Apocalypse gloated.
"Do you ever shut up?" Harry asked in annoyance. "Honestly, shut your c**ksucker and try killing me already! The only good thing about dying would be getting away from your annoying f**king voice! You're an annoying wanker!" Even Charles was struggling to not find that rant amusing. En Sabah Nur talked entirely too damn much.
Thankfully, Apocalypse took the offered advice and shut the hell up and began trying to transfer his essence into Harry. They met in the Astral Plane for their battle and Harry began using his Reality Manipulation to rapidly drain Apocalypses abilities into himself; targeting his regenerative powers, strength and durability, telekinesis, and his other abilities. Harry was going to bleed the annoying bastard dry!
"No Horsemen here to help you. You're finally all alone." Harry said, pleased the pest had finally fallen into the trap.
"You think that will save you, boy?" Apocalypse asked mockingly.
"I won the moment you tried possessing me and neutralized your own advantages." Harry replied before blasting Apocalypse with psychic energy, which also allowed Harry to strip even more power from his enemy. "Here, I have all of your skill and that of everyone I've mimicked and only one enemy to focus on! It was only your horsemen that allowed you to live as long as you did!" Harry said, having been continually blasting En Sabah Nur and taking more and more power from him.
Eventually, Apocalypse was forced to retreat from the Astral Plane and resort to aid from his horsemen, but Harry quickly followed him and casually shattered the bindings holding him down.
"Attack the boy!" Apocalypse said, ordering Angel to aid him.
Harry waved his hand and disintegrated the winged idiot when all he did was twitch.
"You want to be running now," Harry said, looking at Apocalypse with a victorious smirk.
Apocalypse grimaced and fled to get aid from his other horsemen. Harry smirked, aged himself up for longer arms and improved fighting ability; becoming physically sixteen, and then picked up Charles and flew straight through the palaces walls while cutting through them like a knife through warm butter. Naturally, this scared the hell out of Charles and when they exited the palace, his students and the horsemen weren't much better off.
"Don't ever do that again!" Charles said in shock and fear.
"Don't tell me what to do, old man!" Harry said in annoyance. "I had to beat the annoying bastard here!"
"Professor!" Jean said, arriving near them followed by other mutants who's powers were quickly mimicked; except for the guy with an uncontrolled ability and the blue guy. It was Jean's seemingly limitless powers that strengthened Harry the most though, and Harry even sensed another power within her but could not mimic it.
"Charlie, you've been holding out on me," Harry said with a smirk. "Please, introduce me to this embodiment of physical perfection...." Jean blushed at that remark.
"Hadrian, meet Jean Grey. Jean, meet Harry, a five-year old with shape-shifting abilities who accidentally discovered puberty as a toddler and now regularly flirts with women." Charles said, much to Harry's annoyance.
"I prefer the term ageless. So don't worry about meaningless numbers and just focus on what you see." Harry said, but he was sorely tempted to drop the git for sabotaging his attempt at flirtation. "Plus, giving you agelessness is well within my power even if that is an issue."
"Focus... Apocalypse will be arriving soon." Charles said reprovingly.
"And will get his ass kicked," Harry replied. "I nearly bled him dry of his powers and he's extremely weak. The only reason I didn't fight him there was because the palace could have been brought down on you. I'll go talk with Storm and Psylocke and see if I can sweet talk them into aiding us though."
"Were doomed," Charles deadpanned, to his students amusement.
"That's the spirit! That can do attitude will see you living a very long and happy life!" Harry said, before flying over to Storm and Psylocke. "So... ladies. Apocalypse is severely weakened and will be easily defeated, and I played a very big part in basically crippling him. Psylocke, I'll give you agelessness, regeneration, and an enhanced physical condition. Storm, I can give you the same, but you can become ageless when you reach your prime. I'm also serious about wanting to date you both in the future, if all parties involved are single."
"Deal," Psylocke agreed. "No flirting until your legal though. It gets annoying after a while."
"Where is Archangel?" Storm asked in concern and having already suspected his death.
"Apocalypse ordered him to attack and then ran when he was beaten," Harry replied. "He doesn't care about you Storm, or any of his followers. Everyone is a pawn to him and their only purpose is serving until they die. Plus he tried possessing me, a five-year old.... Did that really not bother you?"
"Yes, of course it did!" Storm said defensively. "I'm sorry. I allowed myself to be manipulated."
"I forgive you, but you should still feel free to date me in the future." Harry said, and Psylocke rolled her eyes at Harry's antics. After using his powers to imbue Storm and Psylocke with the specified abilities, they went to join Xavier and his students.
"I was half expecting them to finally castrate you," Charles said in faux shock.
"First of all, it would regenerate or I would use my powers to force it too. Second, they agreed to date me when I'm legal and I gave them eternal youth to bridge the gap between our ages.... So, like I said, I wore them down eventually. You can be in awe of my skills later though, Apocalypse is nearly here. He's been stripped of his regenerative ability, teleportation, Power Absorption, half of his Telekinetic ability, Matter Manipulation, strength and durability, and Power Bestowal and Augmentation. So, should be an easy win."
"He was more powerful than I thought, but you weakened him greatly Hadrian." Charles said in admiration. Harry had managed to trick his enemy into making a crucial mistake and nullify his only real advantage, so Charles had no problem acknowledging Hadrian's intellect and strategic ability.
"He still has his most dangerous ability... annoying people with the sound of his voice. It made me want to die a couple of times." Harry said, to the amusement of most of the newly combined group.
Apocalypse exited his palace with the strange architecture moments later and was promptly pinned to its walls.
"The last few minutes of your life are really gonna suck, just in case you were wondering," Harry said, before blasting Apocalypse while siphoning his powers even further. Harry deemed it wasteful to not do that and he really wanted to take all of En Sabah Nur's power for himself. It was a very dangerous world after all, and Harry was prone to doing stupid things out of boredom.
The X-Men and horsemen joined in, with Magneto finally defecting from Apocalypse and aiding in his defeat, but that armor of Apocalypses was too durable and it took Charles urging Jean to tap into her full power to finally destroy it, which shocked the hell out of Harry in the process. Harry recovered from his shock quickly though and he and Jean finally destroyed En Sabah Nur... forever. As planned, Harry gained all of the annoying bastards power for himself, and was extremely pleased by that. Harry was now among the most powerful mutants in the world with recent events having went to his favor and had even gained knowledge of several more powerful mutants from Psylocke's mind, but that was hardly the best thing that happened to him.... Harry had apparently mimicked the abilities of a Reality Manipulator at some point in time and he had just recently discovered that. That ability was being the primary reason that he survived and became so damn powerful!
"I'm tempted to restore Cairo, but I'm also opposed to resurrecting the dead due to the many issues it could cause," Harry said thoughtfully.
"Then just restore Cairo. If you can, you could even undo all of the damage caused to this world." Charles said, and Charles was hoping that he could. That would greatly improve relations between mutants and non-mutants.
"I'll try, but I'm not giving them back their nuclear weapons. The world is far better off without them, though I'm certain that they will soon begin rebuilding their nuclear arsenals instead of focusing on energy sources to better the world." Harry sighed at that depressing comment and then restored Cairo in an instant by merely willing to be and discreetly hid the objects Apocalypse used to transfer his essence.
"Incredible..." Magneto said in awe. "You could do almost anything!" That caused Charles and his allies to become quite concerned that Erik would exploit Harry to achieve his goals.
"Yeah, but that would get boring really fast. I need to begin repairing the planet. Can you all get back to America or do I need to teleport you all?" Harry asked.
"Teleporting would probably be faster," Charles replied. "The plane they arrived in could be tracked back to the school."
"Drop me off in London," Psylocke replied and Harry nodded.
"I would like to join your school," Storm said, and Charles was all to happy to welcome her in.
With each person having decided their course of action, Harry teleported them all to London to drop off Psylocke, and then Westchester, where Harry restored the school with laughable ease.
"Where will you go, Hadrian?" Charles asked, with them having retired to his office.
"Back to England to clean up the mess Apocalypse made of my life, I'll get my schooling out of the way, and then start working on my revenge plot." Harry replied, getting a disapproving look from Charles for him still intenting to seek revenge. Harry didn't care. His course was set.
"You're welcome to stay here and learn to control your abilities and get your education here," Charles offered, hoping to add Harry to his X-Men.
"Thank you, but no. I'm far to independent and free-spirited, and I'm already very advanced in controlling my powers." Harry would derive absolutely no benefits from attending the school and already had his own plans to work on.
"You could be near Ororo..." Charles said leadingly, resorting to mild manipulation.
"And Jean. She really needs a capable Telepath and Telekinetic to teach her, and she is quite beautiful and needs someone to date when she outgrows her strange attraction to goggles." Charles looked irritated by the insult. "Don't try manipulating me again.... Or I can always make you believe that you're a Catholic school-girl who likes being publically spanked daily while being called a naughty girl."
"Your point is made," Charles conceded with an amused smirk despite the threat.
"Before I leave, I need to mention the entity who has taken Jean as a host...."
"What?!" Charles said in shock.
"Calm down! It's nothing to be concerned about! It's had several hosts and if anything they were better off with it." Charles calmed down. "It's called the Phoenix Force and is the nexus of all psionic energy and is part of the natural order that cultivates the universe to its full potential and guides the natural process of evolution. Jean can become nigh-omnipotent in the future."
"How can it be suppressed?" Charles asked in concern.
"How can a nigh-omnipotent and possibly nigh-omniscient entity be suppressed?" Harry asked, looking at Charles like he was an idiot. "It can't. Trying to would be understood as an attack and the attacker would be squashed like a bug. Furthermore attacking a neutral entity would only cause problems where there are none and Earth could get caught in the crossfire. Jean gets a power boost and Earth gets a nuclear deterrent while it evolves to match other worlds."
"Perhaps some limiters?" Charles suggested, and Harry slipped into Charles mind discovered that he was interested in suppressing the Phoenix to 'protect' Jean.
"Perhaps you should avoid taking risky action where near omnipotent entities are concerned and put in the effort.... Given your concerning suggestions, I'll be informing Jean of this before I leave. She's the one who would suffer the most if you took these dangerous measures."
"No! You will not! I'm responsible for her well-being and I'm merely looking into other options!" Charles said defensively. Charles felt her losing control of her power was too great of a risk, but failed to understand that his actions could cause that. Harry would be as ignorant as Charles was and probably be tempted to make the same mistakes if it wasn't for reading about the Phoenix Force in the Peverell Library. Apparently, a Peverell was a Sorcerer Supreme at one point and copied some secret knowledge and compiled his own.
Harry naturally decided to ignore Charles and tell her anyway... while trapping Charles in his mind. It worked though, and Jean greatly appreciated Harry telling her and Harry even offered to train her in Telepathy and Telekinesis, which Jean accepted. With Harry having no more reason to stay, he returned to Britain.