Natsu had absolutely no idea what happened.
The last thing he knew, he was fighting against Zeref in the Fairy Tail guild hall. It was a tough battle where Zeref goaded Natsu into doing whatever possible to kill him. Alas, even with the powers of END and Igneel, it was an impossible task. You can't kill an immortal. That would go against the very meaning of the word. The curse of contradiction was just that brutal.
Once he got the fairy heart, it really was all over. No amount of fighting could stop the impossible. That's where Natsu should have died, witnessing the end of the world as 400 years of history got erased and rewritten. Instead, he remembered what Zeref said to him in that moment.
"Now that I've finally gotten my wish, I have no more need to destroy you Natsu. Once I go back, I'll deal with Acnologia before he becomes a threat, and finally earn my death. So, before I go, I have a little gift for my brother. Instead of disappearing with this world, I'll have you, the Fairy Tail guild, and my remaining spriggan 12 sent to different worlds where you can live on. I have my peace, so may you find yours too." With that, he walked through the front door. Mavis Vermillion tried to run into the door, but Zeref never caught her eye. As he walked and disappeared, the world flashed white. Everything around Natsu was gone, and he fell asleep.
The next time Natsu woke up, he was in an unfamiliar place. Natsu knew this was some other world, it was somewhere Zeref sent him to survive. So, what to do first. Find somewhere to live and fight off whoever interferes, obviously.
That's when Natsu realized the first issue with the plan. He couldn't feel any powers inside him. Traces of END, bits of Igneel, any dragon slayer magic, it was all gone. He tried to summon a flame only for nothing to happen. There was no magic in his body and absolutely no ethernano in the air.
"You've gotta be kidding me. There's no magic at all. Did I end up in Edolas or something." That's the only option Natsu knew about that had absolutely no magic and thus could stop him from summoning even a small flame. Without magic, Natsu's fighting ability is nearly completely gone. He relies on using fire around his fists to generate power. In a world of no magic, his strength is that of a well-trained human with good instincts. Really good at hand-to-hand, but he'd lose instantly the second someone like Erza pulls a weapon. These were his thoughts in the first hour of landing on the world he'd later find out is called Earth.
As for where exactly in this world he's in, he's not completely sure. He set up a small cabin in the woods where no one save a few deer came. Even in that isolation, he did find a few hunters speaking the same language he could understand. Apparently, he was in the southern end of a country called Japan, in a continent called Asia. He filed away that knowledge. For now, there was a cabin to keep working on until it could somewhat look like his old Earthland home.
That's how Natsu's life went for the next month. There was a rhythm to it, though this life was too boring for Natsu's standards. With no Fairy Tail guild to cause mayhem in, and no magic to center his life on, he was lost. This world was too civilized by what he heard. His only skill is fighting, and there isn't any place for a physical fighter in this world. No one even used swords anymore. Apparently, hunters used guns, which isn't something he can beat without magic. If all soldiers use that, he couldn't really fight. That's how Natsu ended up living the life of a hermit, drowning away his grief at the loss of Fairy Tail.
"Hey, open up in there!" A voice called out. "It's the police. We've been receiving reports of a squatter in the woods, so if you're in there, open the door before we force it."
"Ok ok I'll open the door," Natsu said. He opened the door to be greeted by police holding some weird type of gun he hadn't seen before. This one looked a lot scarier than any gun Bisca or Azlack would wield.
"So, what're you doing out here all by yourself? This isn't private land, it's government property and you just put up a cabin without permission?"
"No one else is using the woods, so why can't I?" Natsu asked, genuinely curious. It's like he never considered that empty land isn't automatically free to use.
"It's government property, which means the government can use it. Since you're breaking the law, I'll have to take you into custody. Do you have any ID?"
"No."
"Really?" The cop said, not believing it. "Nothing at all. Any other documents? Birth certificate, immigration, passport, anything."
"I don't know what any of those are. Why would need to certify your birth, and what's a passport anyways."
"You've really spent your whole life under a rock, haven't you? No matter, there should be a record somewhere in the system. Everyone born in a hospital has a certificate made.
"I don't think I was born in a hospital."
"Ok, at least tell me where you were born, maybe the record is out there somewhere."
"Sorry to tell you, but I don't know where I was born or my parents' names either."
" … Ok just give me your name. I won't get any better." The cop was completely frustrated with the mystery behind this man.
"Finally something I have, my name's Natsu Dragneel."
"Ok then Natsu Dragneel, I still need to arrest you for trespassing on government property, we'll see if we can find any information on you. The cop wrapped him in handcuffs, and Natsu knew that with no magic, he had absolutely no way to fight his way out. He could only hope for the best.
A few days later, cops were looking over the results of the inquiry on Natsu Dragneel. They had discovered that not only did he have no record of existing in Japan, no other country had a record of him as well. Questioning only led to the name of a now-dead foster parent Igneel who quite literally raised him in the wilderness. Because of his lack of documents, he was technically considered stateless. He at least seemed to understand Japanese fluently and if his hair color was a bit more natural instead of whatever pink dye he used, he could probably pass as Japanese.
A few days later, the cops went back to talk to Natsu. The explanation was pretty simple. Because of the fact that he has lived in the wilderness and has no record of existing, it's likely he was living in Japan to their knowledge, and given he already speaks the language, they have no choice but to give him citizenship, mostly because being stateless is against the United Nations charter and a bad human rights violation. Next, they called in immigration to find out what to do with him as far as careers go.
"So, Natsu, what exactly are you good at?" The officer asked.
"I'm pretty good at fighting if that helps."
"I'll kindly ask you not to get into any fights. Though, if you are interested, there are professional fighting competitions hosted by a company called the UFC. Become famous enough there by winning fights and you can make good money.
"Ok, that's what I'll do."
As soon as he was finished and left for the forest, Natsu said aloud, "How do I do that?" He was still very new to this world, with no idea of how to do anything. It was clear this world of Earth doesn't have the bad guys running around like his home did, and in battles, numbers seemed to be everything instead of how strong each soldier was. Since professional militaries were so good, the only other people that knew how to fight were entertainers fighting in competitions for money as prizes. Fighting anywhere else it seemed would get you quickly dragged off to a jail, and Natsu would prefer to stay out of those. He didn't have gramps to bail him out of trouble here.
In the end, it took a month before he ended up signing up at a local gym that had fought in an amateur league. He was clearly the best fighter to go through that gym's doors, no one else could stand up to his crazy strength in training fights. Barely another month after that, he was already set for a competition, which the sensei told him was the fastest he'd allowed anyone to compete.
Natsu didn't disappoint and knocked out his opponent in the first round. His opponent seemed too weak to even hurt Natsu. His punches didn't affect Natsu in the least, and every strike Natsu dealt was busting through his opponent's guard. It got worse once Natsu started using kicks too in his standard fighting style.
It took three more fights before someone noticed. Natsu was scarily good at fighting even without magic. His punches and kicks were just too strong, his endurance too good, and his fighting style too unpredictable and fast. By his size, it didn't seem like he should have the strength he had. He didn't look too tall or heavy, it's true. Of course, magic enhanced his body to deal with all the big guys in Earthland, so he was probably the physically strongest human in the world, blessed with the fighting instincts of a dragon and experience from a decade in a rowdy guild and lessons from a dragon. The scouts coming to watch him didn't know all that of course, but they saw his performance and were impressed.
Sure enough, he got into a professional league very quickly. He was making money at last and was somewhat settled down. He also went ahead and bought that cabin in the woods, simply because it reminded him too much of his home on Earthland. Not a day went by went Natsu wasn't thinking about his family. He missed bickering with Gray, getting scolded by Erza, raiding Lucy's apartment, and especially listening to Master Makarov's speeches on ignoring the council. One of the last things Natsu had heard before getting pushed to Earth was that Master had died. He knew he would see his friends again someday, and that's the only thing that kept him going. Fighting here was just a way to pass time and not fall completely out of shape. It was his distraction until that day came when he could reunite Fairy Tail again. Thinking of Makarov was enough to break him down. Even if all of Fairy Tail reunites again, it won't be complete without the master, the one who cared for Natsu and the others like a father. He really was what connected everyone together. That night, after Natsu received yet another promotion in his league, he had only these thoughts in his mind.
The opponents were a bit tougher in this next league. They could actually keep up for a short time before Natsu overpowered them. Main thing was that these opponents tried to evolve to fight against him. They kept their distance and tried to use reach to avoid his powerful strikes, and were aiming for victory on the ground in grappling as opposed to striking on their feet. For all the fights Natsu has been in, he's never really wrestled or grappled anyone, usually most mages fought on their feet where using magic was easier.
As part of being in a professional league, he was working with a professional trainer now. Natsu's trainer was a specialist in grappling and the more technical aspects of stand-up, which greatly countered Natsu's overpowering and fast fighting style. He picked up the new techniques quickly, as fighting instinct was too thoroughly built into his head. Natsu also spent this time working on more techniques, which is something that kind of gets ignored by mages once they pass a certain power level. At some point, power was so high that slipping and rolling punches just didn't work, especially when all kinds of nasty elements were coating those fists. This magicless time meant his technique was now better than ever, which would only make Natsu that much more deadly should he have access to magic again.
Natsu's fourth professional fight was taking place 6 months after his first promotion, and around 8 months after his first appearance on Earth. He would have been okay with fighting more often, but this was the most anyone was willing to give him. He had been making a real name for himself, and it would pay off here. Scouts from the UFC were present, and this fight would be televised. Ready to show everyone what he's got, Natsu stepped into the octagon.
"In the other corner, standing at 172 centimeters tall, weighing in at 60 kilograms, fighting out of Tokyo, he's the rising star everyone's wanting to watch with fiery passion and power in his punches. It's the Salander, Natsu Dragneel!"
That was definitely the most hyped-up Natsu's ever gotten for a fight. Now, he just had to wait for everyone to clear out, leaving just one enemy in front of him. His opponent was a bit taller and had a bit more reach, but Natsu wasn't at all concerned. It's not like any of them could match up to the bad guys of Fiore.
The first round started with a bell, and Natsu started by leaping forward with a knee into a three-punch combo, slipping around his opponent's counters and landing the first hit of the fight straight to his opponent's chest. What most fighters could take meant nothing when facing Natsu, and his opponent was already staggered from the first connection. His opponent was careful to hold distance after that, only striking when he had a chance to take the fight to the ground. Natsu used all his new training to work his way on top of his opponent, but the bell rang before he got any strikes in. He still clearly won the round by actually hurting his opponent with punches.
The second round started with a bit of deadlock until Natsu started using kicks. He was able to jump long distances to deliver flying kicks, all while having the reflexes to protect himself when airborne. It's stuff the scouts have never seen before. It didn't take long before Natsu landed a kick to his opponent's liver, knocking him down. He quickly dove on top and delivered powerful punches back-to-back until the referee couldn't take any more. That was it. Once again he showed why he was such a feared fighter. Knockout in two rounds.
The result of the fight was like he expected. He was invited into the UFC, the top fighting league in the world for mixed martial arts. It didn't really feel that big to Natsu, but this world treated it like a big deal.
That night, he thought he finally had his new life made out. He genuinely enjoyed fighting competitively against people that weren't evil and also trying to be the strongest they could be. It was more fun than the fights he had as a member of Fairy Tail. He was finally used to this new life. Of course, that's when it all changed.
The next morning, a portal appeared in front of him. He could actually feel magic coming through it, the first magic he felt since that first day. Through it, he saw two people. One looked a lot like Lucy, staring straight at the portal. Now that he took a closer look, that was Lucy. She actually figured out a way to reach him! Amazing! The other woman looked more sinister and was in the process of attacking Lucy. That wasn't something he could just forgive. Without thinking, he put his new life behind him and jumped through, taking aim at the person who would dare bring his best friend harm and shouted Fire Dragon's Roar, releasing an inferno from his mouth.