No author's note, just going right into it!
"Zehahahaha!" A big, burly man with comparatively smaller arms and legs appeared. A young man with a bead necklace stopped in his tracks upon hearing the laughter. "Teach…" He growled out.
The magma man that he was approaching looked up with a scowl. "Blackbeard…. What are you doing here now? Your Shichibukai (Seven warlord) status has been revoked!"
"Well you see, I just got back from Impel Down and I met some new friends!" The now named Teach, or Blackbeard, laughed as his crew appeared behind him.
A giant of a man, standing at 21'10" with a white mustache and a hole in his upper chest, growled. "Teach…." 'Something is different about him.'
"Heya Pops! How's it been? Seems like you've been having a fun time here! Care to meet some of my friends?" He let out his darkness and little creatures of yellow eyes popped up from the dead bodies of pirates and marines. Everyone's eyes widened at the sight of them.
Pops, better known as Edward Newgate, or Whitebeard, scowled. "Sengoku! These creatures are different! We'll have to put aside our differences for the time being to beat them!" The man he was speaking to, a man with spectacles and an afro, narrowed his eyes. "I think so…."
A pale man with blue flames for hair appeared beside Teach with a small smirk. "Ladies, ladies. Please save the round of applause because the show's just beginning."
A giant roar was heard by the burning ships as Whitebeard's eyes widened in horror. "Marco, get everyone out of here now!" From the shore a giant creature with three heads came barreling through the burning ships.
"Ace, get your brother out of here!" Whitebeard yelled as he slammed his fist towards Teach, white cracks appearing in the air as he sent a tremor towards him. Teach laughed and nullified it with his darkness power.
Hades smirked, "Go heartless! Take them all down!" Ace grabbed Luffy as they ran back through the plaza. "Ace what are these things?!" Luffy yelled as the chaos ran through the island. "No idea, just run!"
Sengoku looked at the chaos with narrowed eyes. "Kizaru! Take out the blue flame guy and Blackbeard! Everyone else, focus your efforts on these heartless creatures!"
Kizaru nodded his head and appeared beside Blackbeard, aiming a kick for his head but was surprised to see the blue flame guy block the kick for Teach. "Ohhhh nooooo." His leg was ripped off as he fell backwards, slowly reforming with his light devil fruit.
'This shouldn't be taking long, this isn't good.' He thought grimly before Hades had a grin on his face with a flame in his hand. "You remind me so much of my brother, one that I hate and loathe with every fiber of my godly being. Goodbye light boy." He shot the blue flame at Kizaru like a flamethrower before it froze.
Aokiji narrowed his eyes as he stopped the flames. "Kizaru, get yourself together. This is no time for our typical behaviors." Kizaru nodded his head with a rare serious expression.
Hades chuckled in the meantime. "Maleficent, stronger heartless my dear." A shadowy glob was sent from the sky, heading towards the ground like a meteor. "EVERYONE TAKE COVER!" Jinbe, a fishman with a karate uniform, yelled as everyone moved out of the way.
In the chaos, Teach stalked towards his prey: his commander and the little brother of his. "Oh Ace…." He sung out as he knocked away marines and pirates heading towards them.
Ace froze up as he turned, seeing the man responsible for all of this. "Teach…" He growled. "Zehahaha, we have some unfinished business Ace… your little shit of a brother punched me on my way down to Impel Down. This world will be covered in darkness, my darkness!"
Luffy grabbed Ace's arm as it started to ignite. "Ace don't! You couldn't beat him before; you can't beat him now!" "I won't let him threaten my family!" Garp ran past them, grabbing both boys as Whitebeard had a white orb around his foot and nearly landed on Teach if the man had jumped out of the way.
"We end this now, Teach." Whitebeard said with narrowed eyes. Teach grinned with missing teeth. "I couldn't agree more, pops." Ace tried to squirm out of Garp's grip. "Damn it gramps, I can't leave him there! Father!"
Garp kept his grip on Ace. "Damn it Ace I'm not going to lose you or Luffy!" He yelled, causing both to be surprised by his actions. "I don't know what the hell is going on, but this wasn't supposed to happen, nothing since Teach arrived has gone as either side planned out."
Luffy whimpered, much like he did when he was a kid. "Gramps, I'm scared. I just wanted to save Ace, what's going on!" A gunshot was heard as Garp grunted, tripping and dropping the two young men.
"Grandpa!" Luffy shouted as Garp had his knees shot out by a pirate with a pink hat and a large feather in it. This was Captain Hook. "There is no escape from the heartless!" Garp nursed his leg before standing up, putting minimum weight in it. "Go boys!"
Now it was Ace's turn to hold Luffy back. "I'm not leaving you behind Gramps!" "Go Luffy!" Garp shouted before launching forward as Hook summoned heartless to block the attack.
Sengoku scowled slightly. "Everything's gone to shit real fast." He went into his Buddha form and annihilated a swarm of heartless. Sanjuan Wolf smacked Sengoku against the execution platform, what was left of it that is.
"Zehahaha! Good work Wolf!" Teach laughed before dodging a fist from his former captain. "You were never this strong Teach. What bargain did you strike to gain this power?" Whitebeard demanded.
"Wouldn't you like to know pops? We devil fruit users made a pact with the devil when we ate our fruit, I just happened to take mine quite literally zehahaha!"
Ace and Luffy had made it to the frozen ice of the bay of Marineford. "Jinbe!" They called out as the fishman was fighting off heartless as he saw them. "Luffy! Ace! We got to get out of here!" He joined them at Whitebeard's hidden ship emerged before tentacles wrapped around it, breaking the ship apart and dragging it to the bottom of the sea.
A maniacal laughter was heard as a giant lady with the bottom half of her body being an octopus surfaced. "Fools! There is no escape for any of you! Your world is doomed!"
More heartless came and surrounded the three. Luffy looked around in worry as his rubber fists went through them and more took their place. Ace growled before lighting up a flame circle around them. "Luffy, I got you! Enkai: Hibashira!" He summoned forth a giant flame pillar that protected them as the heartless rammed themselves against it to break it.
"Jinbe I can't hold them off forever!" Ace said while gritting his teeth. "I'm still weak from being held prisoner." Jinbe readied his stance. "It was an honor to fight among you three." A giant sword slash cut down the heartless as Ace's flame pillar held up barely.
"Hawk guy!" Luffy exclaimed as Mihawk landed beside them. "Go Straw Hat. I can hold them off." Luffy nodded and ran off with Ace and Jinbe.
Meanwhile, in other places of the world, the heartless were attacking. Not just Marineford, but Alabasta, Mariejois, Fishman Island, and everywhere across the seas. Zoro grit his teeth, fighting them off before succumbing to the void. Sanji complained about not being able to rescue the ladies before he fell. Robin and Nami both called out to Luffy before the darkness overtook them. Brook, with his last skull joke, fell. Chopper took down many heartless before a giant one collapsed onto him, taking down the little reindeer doctor. Franky annihilated as many as he could before he was overwhelmed, his last words being "not super." Usopp was blocked off but went down like a brave warrior of the seas.
The old men of Marineford, Garp, Newgate, and Sengoku were back to back as they were surrounded. "Garp, Sengoku, I never thought I'd go down fighting alongside the two of you."
"I didn't expect this at all. Done in by a bunch of shadow creatures. I thought Garp would've given me a heart attack before going down like this."
Garp remarked snidely, "Gee thanks Sengoku." All three men roared as they unleashed their full powers and brought down the island of Marineford with them, or at least planned to.
Luffy bounced off the ground away from the heartless. He looked up and found Ace being swarmed by them. "Ace!" He shouted desperately as the Mera user tried to ignite his body to burn them off of him but found he couldn't. The heartless surrounded the both of them as Luffy reached out for him.
"Luffy!" Ace yelled as they barely touched fingertips before both boys found themselves in a world of darkness.
So yeah… this will be tough to explain, but I all of a sudden had this idea pop into my head doing some reading. Thanks to Kfbanime87 and his Shinobi of Twilight for this story of idea. It seems like I'm really not giving much effort from the One Piece characters in their fight against the Villains and heartless, but let's be honest: Marineford was a very tiring battle for both sides. Whitebeard was nearly dead, Akainu was spent, and Sengoku and Garp can only do so much before they too are eventually overwhelmed. Combine that with Teach, his crew, all enhanced by the powers of Darkness, as well as Maleficent, Hades and Ursula being there as well, you can quickly see why what happened, happened.
Now, with that being said, I left a lot of stuff open-ended as to who could be dead or who could be alive. Akainu is a rough spot for me because I love to hate him, but I don't know if I should make him one of the main villains alongside Teach with the villains group or have him killed off. Another rough spot is who do I make the main character? Do I make it Ace or Luffy? Or have them both go through it? This is a brand new territory for me because I know One Piece a decent amount, but I do not know Kingdom Hearts very well (time to binge play that, hooray!). So moments after this story goes up, I'll be making a poll as to who the main character should be, and by that I mean who gets the keyblade? Leave a review as to whoever should be alive, who should be dead, who should be corrupted, yada yadda, stuff like that. I actually did have a lot of fun with this idea, despite it being very short as an introduction.