Premise: Toothless is unable to come to the rescue, and Hiccup dies in the ring with the Nightmare.
I don't own How To Train Your Dragon and no profit is being made here.
Astrid keeps her promise, even though he failed to keep his: nobody ever finds Toothless.
She almost doesn't go back. Almost. But though trying to forget she even knew there was a Night Fury in a secret cove in the forest would technically be keeping her promise, Astrid is pretty certain he wouldn't have wanted his pet left there alone to die, and that doing so would be just as bad as leading the village's best dragonfighters right to him.
That, and she witnessed that moment, the moment when the Nightmare looked as though it would stand down and let him touch it. She saw. That moment of calm and curiosity, right before its slit-eyed startlement at the clang of Stoick's hammer against the cage. And she realized, He's right. Toothless is not a fluke. We don't have to kill them.
It is ironic that the first Viking to befriend a dragon was still killed by one. Astrid has never had much appreciation for irony.
Years later, she'll still have nightmares, and wake up with his horrible screams and the sluggish crackle of liquid flames ringing in her ears, while her husband grumbles and rolls over beside her.
His secret is almost more than she can deal with. Everything is changed. Astrid hangs up her axe and shield, and asks to be apprenticed to the healer. At first everyone thinks she is joking, then they think she is traumatized, and when the joke doesn't stop and the grieving period is over and she still refuses to go back to the ring, nobody is sure what to think of it.
She almost doesn't go back to the cove, but she does, and it breaks her heart all over again. The most fearsome dragon that Vikings know of is huddled in a sad little ball at the base of the cliff. At the sound of her footsteps he immediately bounds up, then just as quickly deflates when he sees she's not him, spilling back across the grass like mead from a broken jug.
The cliffside is scarred with deep gouges. Gleaming black scales are scattered everywhere. Toothless's claws are cracked and broken.
The dragon won't let her touch him, but while Astrid sits on a rock and cries it curls up next to her and keens, and she knows he knows.
Astrid doesn't know how the strange flight rigging he made works, or if Toothless would let her fly him even if she did.
She sneaks into the forge at night before anyone else thinks to, because they've lived on a very tiny outcropping of rock together since birth and she's seen him with various incarnations of the little notebook a thousand times. There's sure to be something about Toothless in it and therefore nobody else must see it.
Sure enough, there are dozens of sketches of the dragon, various designs for the rigging in several stages of completion, and some rambling observations about the mechanics of flight, along with similar notes and scribbles for several other projects. Leafing through it, Astrid realizes that despite living cheek to jowl in this tiny village with him her whole life, she really didn't know him at all. Didn't even really meet him until less than a week ago. And now it's too late.
The boy she grew up with was distracted, which made him clumsy; both of which made him a nuisance. Peering through this dim window into the sorts of things that were distracting him all that time is…it makes her feel like the clumsy one. He was smart. Even after examining all the drawings, she still can't figure out the rigging, and eventually she takes it off the dragon so it won't chafe him.
She has to wonder at the sort of fearlessness it must have taken to even think of putting a saddle on a dragon in the first place.
Toothless allows it, crooning morosely. Watching him plod miserably around the cove with half his tail missing hurts more than she thinks it should.
After a few weeks he's warmed up to her more, will let her scratch behind his head. It is almost possible to see Toothless as an entirely different creature than the dragons locked up in the training ring, and that descend on Berk at night to raid. Almost.
Astrid is no dragon tamer. She could try to show the others what she learned from him, but that might mean breaking her promise. And then she thinks she could tame a dragon in the ring, just like he died trying to do (though it had worked before, hadn't it, wasn't that what he had done all along in training?), but Astrid is no dragon tamer. Then she thinks she could tell the others about the mountain in the fog and the horrible beast at its heart, but how would she explain how she knows?
Keeping her promise is burying everything he tried to prove.
For a little over a year Toothless hangs in with her, crippled and mourning, her big dark secret in the woods, and there are whispers that she is starting to go just as strange as he was. Toothless dies in the spring. There is snow on the ground, and blanketing the unmoving black body in white.
When she's cried herself empty, Astrid returns to Berk with a pocketful of night-black scales, the weathered notebook, and the saddle.
Toothless is safe now, just like he wanted. Now it's time to set things right.