This story is going to deviate from the movie at a very common point. A single decision by Hiccup changes everything. Standard disclaimer: I own nothing.
Hiccup was the only thing standing preventing a full blown fight that would certainly lead to bloodshed. The black dragon could have easily knocked him to the side to get to the girl Hiccup was protecting, yet it stayed respectfully behind the boy. "She's a friend," he told the great beast and to the girl's surprise, it calmed down. "You scared him." Hiccup told the frightened girl as the dragon made another halfhearted attempt to get past Hiccup.
"I scared HIM?" It was an impossible situation. She had concentrated so much on the dragon in front of her that she never expected Hiccup to blind side her and rip her axe from her hands and toss it away. Now defenseless, the scrawny boy was the only thing between her and being ripped to shreds on midnight black claws. Then her mind caught up with her situation, "Who is 'him'?"
She could never quite appreciate Hiccup's sarcasm. It was way of dealing with an uncomfortable situation, but for the village screw-up, that was most of the time. And he did it now. Standing up straight, calm as you please, he made introductions, "Astrid, Toothless. Toothess, …Astrid" The dragon hissed menacingly at her but made no move against her.
Astrid was so focused on the night fury that she missed the nervousness Hiccup displayed at the dragon's continued aggression. A lot of small events suddenly made sense to her; how the dragons seemed to fall at his feet or how the scrawny screw-up seemed to take them down so fast without ever injuring them. He was working with the dragons the entire time, not against them. And if he was with the dragons that meant he was against Berk and the Hairy Hooligans. She turned and ran, gambling that the boy still had some decency left in him to not attack her from behind.
"Dun, da-daa, We're dead." Hiccup said to no one in particular. The dragon seemed pretty pleased with himself since the intruding female left, satisfied that he had scared her off. He turned and went back to the baskets his rider had brought. "Whoa, whoa, whoa. Where do you think you're going?" Hiccup took one last glance in the direction Astrid had run just in time to see her scoop up her axe as she ran. She didn't even look back.
"Come on, Toothless, we've got to stop her. She could ruin everything." Hiccup said as he ran up to the dragon. Toothless picked up the half-filled basket and dropped it in front of the boy. It held some clothes and a few provisions; just enough to help him survive once he left. That was his original intention after all. He could still leave. Besides, there was no way Astrid could return to the village, tell her story and bring someone back that could stop him, after all.
Plus she had her axe. One good throw could cripple or even kill Toothless, not to mention himself. Trying to stop her would not end well. The only choice left was to leave and never return.
"Yeah. Come on, bud. We have a long flight ahead of us." Hiccup stowed the rest of his things in the basket, including some tools and extra metal rods for Toothless' tail. Once he was set he took off with Toothless, soaring over the island that was no longer his home.
Dammit, he didn't want it to be this way. It would have been better if simply disappeared, but now with Astrid going to tell her story, he would be branded a traitor. He couldn't return if he wanted to. That was what really bugged him. He knew, logically, that he was leaving forever, but it always seemed that there would be some way back, even for a visit. Just to let his dad know he was all right. Now, with Astrid telling her version of everything, that was no longer an option.
Hiccup was jerked from his thoughts as Toothless went into a sudden dive. Surrounded by fog, rock pillars jutting up from the ocean appeared in front of him, he had a pretty good idea where he was. Helhiem's gate.
"Bud? What is it? What's going on?" But Toothless didn't respond. Changing the tail angle would do little good now, or worse, it could send them crashing into the stone columns that raced by them.
Suddenly, another dragon appeared to his left, a monstrous nightmare carrying a wild yak. Another appeared, a nadder carrying a large fish. Others appeared to his right. Then more behind him and above him, all of them carrying some sort of food item. And he was right in the middle of all of them.
"Toothless, you got to get us out of here." Hiccup touched the dragon's neck to get his attention, but Toothless shrugged him off and continued with the dragon flight. Like it or not, Hiccup was along for the ride. He flicked the tail as great black rock columns appeared suddenly out of the fog directly in their path. None of the other dragons seemed to have a problem navigating this deadly maze either. He had no choice but to simply follow along.
Ahead the fog cleared as a large volcano loomed in front of them. The dragons didn't slow as they dove into a cave leading to the glowing interior. Toothless finally broke from the flight and landed on a narrow shelf behind a stalagmite. Hiccup watched as all the dragons flew by, dropping their load of food into the glowing hole. It made no sense to him, why bring food all this way just to trash it?
The last of the dragons went by until an old, nearly feeble gronkle wandered in and regurgitated a single small fish. That was when something at the bottom of the glowing hole roared its disapproval of the tiny fare. The gronkle seem suddenly frightened and tried to speed away from the hole as fast as its tired wings could carry it. It wasn't fast enough as a giant dragon head shot up out of the hole and closed its jaws around the gronkle. Every dragon in the cavern attempted to make itself as small as possible and not draw the attention of the great beast.
"What the…?" Every expletive Hiccup had ever heard from every Viking and sailor that ever lived in or visited Berk flashed through Hiccups mind at that moment, though none made it out of his mouth. The giant head began to sink back into the hole when it stopped and started smelling the air. The great eye of the beast rotated and fixed on the corner that Hiccup and Toothless were hiding in. "Time to fly, Toothless. Now!" Luckily, Toothless did not hesitate as enormous jaws snapped shut where he had been just a second ago.
The cavern erupted into chaos as all the dragons attempted to leave, some flew into the crevices that lead outside and some spiraled up to the hole at the top of the volcano. Hiccup pulled Toothless into a vertical climb as the giant dragon came after them. The great teeth snapped shut just behind Hiccup again, but a zippleback had flown too close and was now little more than a snack.
Toothless did not stop his breakneck speed away from the volcano that was at the center of Helheim's Gate until he had cleared the fog that surrounded it. Hiccup urged the dragon to land on one for rock formations jutting out of the sea. Hiccup fell to his knees as he caught his breath. He had never been so terrified, even when Toothless had him under his claws in their first encounter. Finally calm he tried to make sense of what he had witnessed.
The dragons fed it. They were scared to death of it but they still fed the giant dragon at the middle of the volcano. Why? The volcano had to be some sort of nesting area for dragons. It made sense; dragons were nearly fireproof, after all. What better place to warm the eggs. If the eggs were there, that meant the giant was using them to keep other dragons under his control, forcing them to bring it food.
"The other dragons, they are slaves to that big one. Aren't they?" Hiccup asked, not taking his eyes off the fog that hid the volcano. Toothless just warbled an agreement. He too was glad to be out of there. He may be tied to a rider, but it was better than the day to day survival inside that fiery mountain.
Hiccup turned to look at Toothless when a glimmer of light caught his eye. A small hint of light on the horizon that Hiccup knew all too well: Berk. An entire village containing hundreds of lives summed up in a single flickering yellow speck.
The moon continued to rise as Hiccup considered it all. But there really wasn't any choice at all, in the end.
"We have to go back" He told Toothless. "I can't just leave them to the mercy of that… thing. I'm the only one who knows it's there. I'm sorry, Bud, but I can't leave them like that." He jumped on Toothless' back and flew toward the flickering light. If the night fury had any objections to the change in plan, he never voiced them.