Here is Astrid's part of the story.

Hiccup's been gone for too long, Astrid thought as she paced in Stormfly's stable, debating whether or not she should go after him. She really wanted to, and had even told him that she'd come and get him if she got worried, but Hiccup had told her to stay on Berk, where she was safe.
But since when did Astrid do what she was told?
She mounted Stormfly, her axe gripped tightly in her hand. She didn't want to say anything to Valka until she was sure something bad had happened. Who knew, maybe he had just gotten distracted by a new island or something?
“Let's go,” she said, a determined look on her face. Stormfly took off and, with a final glance downwards, left the island of Berk.

They finally reached Itchy Armpit, and from there flew to the burnt village.
What was in front of them had Astrid nearly gagging again.
Every single villager, man and woman, had been strung up on burning torches like the first ones. Their injuries were the same as before, meaning nearly every strand of grass was stained red.
“What in Odin's name happened here?” she wondered, as she jumped off Stormfly's back and walked round the now lifeless village. What had once been a peaceful little town was now a field of ashes and the inhabitants had been gutted like fish.
Suddenly, her mind darted back to Hiccup. If this was what the masked man could do to a whole village, then what would he do to him?
Astrid racked her brains, searching for a way they could track him down, when she spotted something she recognised a little further away on the ground. She and Stormfly ran to it and the young Viking picked it up: Hiccup's flight helmet.
She put it in front of Stormfly's nostrils. Deadly Nadders, once thought to be part of the Sharp class, had recently been discovered as Tracker class dragons, meaning their sense of smell were the finest in the dragon world.
“Can you find him?” she asked her dragon. The Deadly Nadder made a happy chirping noise to say yes. Astrid smiled and mounted her dragon again. “Go get him, girl!”

Here is Astrid's part of the story.

Hiccup's been gone for too long, Astrid thought as she paced in Stormfly's stable, debating whether or not she should go after him. She really wanted to, and had even told him that she'd come and get him if she got worried, but Hiccup had told her to stay on Berk, where she was safe.
But since when did Astrid do what she was told?
She mounted Stormfly, her axe gripped tightly in her hand. She didn't want to say anything to Valka until she was sure something bad had happened. Who knew, maybe he had just gotten distracted by a new island or something?
“Let's go,” she said, a determined look on her face. Stormfly took off and, with a final glance downwards, left the island of Berk.

They finally reached Itchy Armpit, and from there flew to the burnt village.
What was in front of them had Astrid nearly gagging again.
Every single villager, man and woman, had been strung up on burning torches like the first ones. Their injuries were the same as before, meaning nearly every strand of grass was stained red.
“What in Odin's name happened here?” she wondered, as she jumped off Stormfly's back and walked round the now lifeless village. What had once been a peaceful little town was now a field of ashes and the inhabitants had been gutted like fish.
Suddenly, her mind darted back to Hiccup. If this was what the masked man could do to a whole village, then what would he do to him?
Astrid racked her brains, searching for a way they could track him down, when she spotted something she recognised a little further away on the ground. She and Stormfly ran to it and the young Viking picked it up: Hiccup's flight helmet.
She put it in front of Stormfly's nostrils. Deadly Nadders, once thought to be part of the Sharp class, had recently been discovered as Tracker class dragons, meaning their sense of smell were the finest in the dragon world.
“Can you find him?” she asked her dragon. The Deadly Nadder made a happy chirping noise to say yes. Astrid smiled and mounted her dragon again. “Go get him, girl!”

After what seemed like a few hours, Astrid noticed that the sea below them had become darker than before, and rocks stuck out of it like teeth. As Stormfly kept going forward, they entered a thick fog.
“Thank Thor I'm not relying on my own eyesight,” Astrid said to her dragon. “You can probably see through this as if it wasn't there!” Stormfly made a happy chirping noise to agree.
Suddenly, the Deadly Nadder stopped and hovered, her eyes darting around the area cautiously. “What's wrong, girl?” asked her rider. “Have we arrived?”
Stormfly swooped downwards slowly and quietly and landed on a lifeless island which had been invisible until now because of the fog surrounding it. There wasn't a single noise except the breathing of the two.
Gripping her axe, Astrid took a few steps forward and saw a cluster of sticks stuck inside the ground. To her horror she saw that on top of these sticks were the skulls of various dragons, including the Deadly Nadder. Astrid immediately lead Stormfly in the other direction so that she wouldn't take fright.
I guess we've found the masked man's island, she thought.
Scattered around the island were what looked like abandoned Whispering Death tunnels, the old occupants having either been killed or fled the brutality of the masked man.
“Let's see what's down there,” Astrid said as she climbed back on to Stormfly's back. The Nadder quietly took off and dived down one of the tunnels, which was about twice her width, meaning she could easily fly down it.
In what little light there was, Astrid looked round the tunnels. Every here and there there was a pile of bones and old rotting wooden doors, which the girl guessed to be unused cells.
Suddenly, voices were heard up ahead. Stormfly quickly landed behind a rock that was jutting out of the wall and curled herself up, so that not a single part of her or Astrid was visible. They waited silently as two masked soldiers walked past the rock, not noticing the girl and her dragon behind it.
Once they had gone, Astrid got off Stormfly's back. “Stay here, girl. In case an unexpected soldier walks by it's hard to hide a whole Nadder, and I don't want anything to happen to you.”
Stormfly nuzzled her as if to say “Don't go!”
“I'll be fine,” said her rider, as if she knew what her dragon had said. “Just stay here, OK?”
And with that, she was gone.

Astrid walked round the tunnels, dodging a soldier every now and then, for what seemed like hours, when she finally came across a solid iron door that was twice as big as she was.
“That must be some sort of main base,” she said to herself, as she pushed it open cautiously.
Behind the door was a whole new set of tunnels, smaller this time, most likely made by people and not Whispering Deaths. From each and every one came a sound that resembled screaming, but there were so many echoes it was hard to tell.
As she was deciding which tunnel to go down, she heard a noise that sounded familiar. She could barely hear it over the other screeches, but it was there somewhere.
She walked in front of every entrance, her ears trying to pick up where the sound was coming from, and finally she heard it again. It was muffled for some reason, but no doubt it was a Night Fury call.
Astrid ran down the tunnel in question as fast as she could, still holding her axe in case she came across someone, when finally she reached a large cage with thick iron bars the width of sheep that was encrusted into the wall. Inside it she saw something that broke her heart.
There was Toothless, looking at her with big sad eyes, every part of him covered in either scars or chains. His prosthetic tail had been removed and he had a spiked muzzle around his face, explaining why the calls had been so muffled and quieter than the other screeches.
“Toothless!” she practically sobbed. “Don't worry, I'll get you out!”
With one strong swing of her axe, she demolished the thick padlock on the cage and walked inside, getting rid of all the other padlocks on the chains. Finally she removed the muzzle, only to have a large wet tongue lick her all over.
Astrid laughed as she stroked his head. “I'm happy to see you too! Now come on, let's find Hiccup!”

Back to Hiccup's point of view.

Darkness. That's all. The second Savage had left the room in fury I'd passed out, thankful to at last be able to rest. I felt as if I couldn't go on much longer, but every time I felt like giving up, I had to remember that if I did, Berk was pretty much done for. A broken chief is like no chief at all.
I sat there with my head hanging, so quiet and so motionless, that if anyone had looked at me, they'd have thought I was dead. Fortunately I wasn't, but I was pretty close.
Despite my unconsciousness, I swore I felt a soft hand touch my cheek. Who was that? Had I died during my sleep and was Odin welcoming me into Valhalla? No, the touch was too soft. Had Freyja chosen me to go to her kingdom, Fólkvangr, instead? (Fólkvangr was the place that half the soldiers who died in battle would go to. The other half would go to Valhalla. Freyja was the goddess of love and beauty.)
I felt the hand leave my cheek and I wished that it would come back. It was so soft, so caring, so reassuring.
Suddenly, there was a loud THUNK and this nearly awakened me fully. I felt the chains around me loosen and I felt myself fall forward without the support, only to be caught and put upright again. Yes, I was surely being led to Fólkvangr. But what was that?
“Hiccup!” said a blurry voice. “Hiccup, wake up!”
That didn't sound like Freyja. That sounded like...
“Hiccup, get a move on or I'll leave you here as well!”
Astrid!