AN: Never ship immortals and mortals. It ruins you.
The light has gone out of my life. - Theodore Roosevelt
Hiccup had never really believed in myths. They were told throughout his village, known word-for-word by the children that heard them oh so many times from the older kids. Tales of characters that gave gifts or stole teeth, or sent the winter.
He only began believing because of Jack.
That fateful night when, as he was riding on the back of Toothless, the dragon's wings had frozen over, and they had fallen through the clouds, unable to break free. Hiccup had thought they were done for, until something – someone – broke the ice. They had survived by only a few meters.
Hiccup would have brushed the entire thing off as an atmospheric disturbance. He almost did. Had the ground not frozen solid underneath him, he would have never known. But it did, and he remembered the figure known as Jack Frost. He had no idea why this particular story would come to mind, but a boy solidified out of the shadows, and he knew.
He was the first to ever have seen the winter spirit – the other children, however much they professed to believe, never quite believed hard enough. He fell for the white-haired boy as hard as they had fallen through the clouds – Toothless could, of course, already see the mischievous sprite, being an animal of great perception, and had befriended him quickly. Jack, in turn, felt love for the first time in his immortal life. He dreaded the summers, knowing that he would have to travel far away from his beloved to bless the southern hemisphere with winter, but he would always come back, his love never fading.
They kept up a relationship through the years, and Jack would never age, and Hiccup would. The winter spirit watched on in silent horror as the boy he loved grew into a man and then into an elder, never taking a wife no matter how much he was pressured. Even his boyhood crush, Astrid, could not hold his attention, and she gave up, seeking out another companion. He allowed his title of leader to be taken through a competition, and became the old inventor who lived in the woods with his aging dragon, only emerging to present his new creations every so often. He secluded himself, knowing they would never accept his wifeless stature, or his lack of any sort of romantic entanglements. He knew his fate. He would die old and alone, but not really.
Jack observed as Hiccup slowly began to fade. He invented less. He omitted some of his flies with his dragon and his partner. He developed a hacking cough, right as winter was creeping on the edges of the region, and Jack knew that he would not last through the cold months.
One night, a few weeks after the date that marked the start of winter, Hiccup called Jack to his bedside. "I am old, and I am dying," he said, a great strain in his old and weathered voice. Jack went to take his hand, and realized that his cold skin would not help much. "I want you to make sure Toothless does not die alone," he murmured, and Jack nodded, unable to speak past the lump in his throat. The dragon in question had his head through the window, looking just as old and frail as his master.
A short puff of air, misting as it left his lips, and he was gone. Like countless humans before him, Hiccup had aged, grown weak, tired, sick. Died, as Jack Frost had watched, as he had kept winter at bay, in hopes of saving him. Tears streaked down the boy's cheeks and froze, turning into snowflakes and drifting to the ground. He pressed his lips to Hiccup's forehead, creating an intricate pattern across his body, effectively freezing him solid. "This is all I can do," he whispered, knowing full well he would not be able to lift his former lover like he used to, complaining about how skinny he was. He gave a sad smile at the memory, and turned away, to Toothless, who had not made a sound since his master died.
"Toothless?" he asked, and the dragon gave no response. "And the captain goes down with the ship," he murmured in a slightly wondering tone, and placed his staff atop the dragon's head, doing the same thing to his body. "But what of the first mate?"
That night, Berk saw the worst blizzard in its history.
Jack never went back to that village. He ignored it, and the memories surrounding it, and convinced himself that it had never happened, he had never met a boy with green eyes and brown hair and a brilliant mind, never fallen in love with him, never watched him die. He convinced himself that no one had ever believed in him. He hid in his hometown and swore never to love again, and distanced himself from everyone and everything, focusing on doing his job, leaving his lake only to give the world winter.
He forgot.