It started with an unbearable itch.
Thorin rolled over on the bed as quietly as he could, the sheet thrown over his bare chest cast aside. He sat up and rubbed the sleep out of his eyes, making sure that the springs in the bed didn't squeak as he did so. The last thing he wanted to do was to wake the hobbit sleeping beside him, Bilbo - his intended had made it clear exactly how much sleep he required after spending an entire day negotiating with Bard and Thranduil.
Bilbo and Thorin had been engaged for almost four months now, their wedding date set in late dabladrân. They were waiting out the long winter of Erebor to have what Bilbo had deemed "a proper wedding" - one that took place outside and followed all of the correct Shire traditions from his culture. Thorin would rather just sign the marital papers and be done with, but his betrothed had insisted, superstitious old fool that he was...
He rose from the bed and pulled his nightshirt over his head, not bothering to smooth out the wrinkles or run a hand through his unruly hair. The terrace doors were already open, a cool night breeze drifting into the bedchamber. Thorin walked out onto the balcony and reached behind him to scratch the irritating itch on his back.
The chilly night air was refreshing, and it soothed the obnoxiously itchy spot on his back. He closed his eyes and listened to the sound of the forges below in the mountain, even audible from the seventh floor of Erebor. A feeling of content settled over him, and he exhaled softly, taking another -
A dagger was stuck right between Thorin's shoulder blades, and he lurched forward, nearly tumbling over the balcony rail. The doors to the bedchamber slammed shut behind him, and he gritted his teeth as the blade dug deeper into his flesh.
"Whoever you are," he hissed through his clenched teeth, "take my life first and then leave. Do not even attempt to kill anyone else that resides in this mountain, or I swear to Mahal I will come back from the dead and kill you myself."
There was no response, unless the drawing-out of the knife and the re-inserting of it counted as a reply.
Thorin cried out and gripped the railing until his knuckles were white. His vision was blurring, spots of darkness dancing across it and further obscuring his sight. For a second, his fingers looked like gigantic, scaly black talons, but then he blinked, and they were gone.
The pain must have been causing him to hallucinate.
Suddenly, the knife was gone, almost as if it had disappeared into thin air. He staggered to his feet, having collapsed against the railing earlier, and exhaled heavily.
His jaw snapped shut again, and he thought for sure that he must have been hallucinating. Was he going mad, or had a snatch of flame exuded from his mouth for a second?
He opened his mouth again, inhaled, and took a breath out, then clamped a hand over his mouth. He was breathing fire - oh, Mahal, how exhausted he must be to be having such a horrible, horrible vision...
Thorin was suddenly jerked up into the air by something behind him - magic? His attacker? Something he was completely unaware of? - and the wind tore a cry from his lips, though it was completely inaudible in the harsh roaring of the updraft carrying him skyward. He arched his back in an attempt to fall back to the ground, and there was a loud snap behind him, reminding him of the sound Smaug's wings had made when he had quickly unfolded them.
The ascent slowed, and he glided over Dale, still unaware of what was going on.
A searing pain then assaulted his squinted eyes, obscuring his vision completely and forcing him to lower his eyelids tightly. The darkness of the backs of his eyelids blinded him for a while, until the pain stopped, and he was able to blink and open his eyes again.
Everything around him was tinted a radiant shade of gold, and his eyesight was blurring terribly. His surroundings were practically swimming around him, and when he looked down at his hands, they were the terrifying, scaled claws again.
Realization hit him like a wall - Thorin was turning into a dragon.
This must be one of the side effects of dragon sickness, he thought. He was strangely calm about it; at any other time, this epiphany would have scared the living daylights out of him, but he was tranquil. He suspected that the cool night air, and the exhilarating sensation of flying, was what was keeping him at peace.
In fact, Thorin might have been completely fine with being a dragon -
Something like a needle pierced his scaleless underside, and he scrabbled at it with his talons, careening towards the mountainside. Thick, warm blood coursed down his chest, and with an astounding crash, he smashed into the peak of Erebor, screaming silent curses in Khuzdul.
Bilbo awoke to an empty bed and a missing dwarf king, which didn't initially alarm him. His intended often went missing in the mornings, attending meetings or taking care of business around the Mountain.
A few hours later, though, after interrogating countless dwarves that couldn't provide him with a proper answer, Bilbo started to get worried.
He went outside.
That was when he found the vast dragon carcass sitting at the bottom of the mountain.
The scales of the great beast had the exact hue of a suffocating mine shaft, dark as pitch and reminiscent of his encounter with Gollum in the Goblin Tunnels. He shuddered, horrified, and went off to find Bard.
After two days, the corpse of the dragon disappeared, replaced by the broken, shattered body of a dwarf king, an arrow sunk deep into his breast.