Thorin emerged from his battle with Azog the Defiler victorious, but broken. His nephews, his beloved sister-sons, whom he loved and raised as his heirs and his own, were dead by the hand of the foul creature who had taken so much of their family. There was some comfort in the fact that he finally avenged his kin, but it was a cold comfort in the face of all the death the Battle of the Five Armies had wrought.
His heart broke when he saw the blade of Azog pierce Fili’s chest, saw his golden-haired nephew fall from that evil’s hands into the ice and snow, but it was shattered when he learned of Kili’s death as well. Despite the overwhelming odds and the ferocity of the ensuing battle, he’d still, somehow, held out hope that the valiant, gentle archer had survived.
It wasn’t until Balin and the rest of the Company had found him with Bilbo in the healer’s tent at the end of the battle that he learned of Kili’s death. None of them could meet his eye when he demanded to know of his younger nephew’s fate, but the red-headed woodland Elf—the one that had so obviously fascinated Kili from their first meeting in Mirkwood—that accompanied the other Dwarves, and the crest-fallen, heart-broken expression on her face had said it all.
Thorin sank to his knees, inconsolable in his grief for almost a week straight, so much so that it clearly frightened all who knew him. The King Under the Mountain had only vague memories of those first days after the battle, everything lost in the haze of pain, but one thing that stood out bright and clear was a small hand rubbing his back as he wept for hours on end and coaxed him into eating during the interim.
When he emerged from his tent on the seventh day, it was to find his ragged-looking Company of ten remaining Dwarves and one especially haggard hobbit, all thin, with drawn faces and haunted eyes. Thorin gazed into the eyes of each of them, Bilbo last of all, seeing his own pain reflected back at him, and nodded.
With his tears all cried and his grief now a constant ache, the new King of Erebor strode out into the rest of the camp with his shoulders squared and his head high. He had much work to do.
The Dwarves of Erebor, spear-headed by Dwalin, had kept anyone but the company from entering the mountain while Thorin was submerged in his grief. He was grateful for it, grateful that his friend had followed his wishes when he could not make any decisions for himself, but rescinded the order immediately. All would be welcome in the halls of his father and grandfather for the coming winter—in his halls.
He paid what he owed to the Men of the Lake, and gifted the new King of Dale with even more treasure, enough to keep his beloved family in comfort for many generations to come.
The Elves had taken their leave of the battlefield once the Orcs and Goblins had been defeated, Thranduil and his army running down the rest as they made their way back to their accursed forest. Only two remained: the redheaded she-elf with more love in her heart for Thorin’s dark-haired sister-son than he had at first suspected, and (surprisingly) Thranduil’s son, who seemed to be… escorting his companion.
Apparently having had their fill of violence, the two stayed to help rebuild and get both Dain’s forces and the Laketown refugees settle into Erebor. The Prince—Legolas Greenleaf, evidently—seemed anxious to move on, but the woman, Tauriel, seemed to want to stay, leaving the mountain each day to hunt down any straggling enemies and returning always with small injuries as well as meat to feed the voracious appetite of Erebor’s residents.
After many weeks—after the beloved princes of Erebor were entombed in stone as was custom, watched by a stone-faced and dry-eyed Uncle and an equally stoic mother—Tauriel finally approached the King Under the Mountain. While regal and still carrying the same unconscious arrogance as all other Elves, she walked up to his throne without any weapons and knelt before Thorin in front of many a spectator and, in a resounding voice, begged his forgiveness for having failed to save his nephews.
Shocked as he was, it took a moment for Thorin to react, and before his mind processed everything, the she-elf continued. She spoke of how, while she had met them as prisoners and she as their jailor, they were brave and loyal and kind far beyond what the situation called. (The last part uttered so softly that Thorin wasn’t even sure she had meant to utter those last grief-stricken words aloud, and was struck by remembrance of Kili conversing with an unseen feminine voice before Bilbo had arrived to free them from Thranduil’s cells, recounting a tale of a Blood Moon and promises. This was she, he realized, who had walked in starlight and laughed at his silly nephew’s jokes and commended the pure love of his promise to his mother.)
When Tauriel finished, ending with the offering of her blade and bow to the King Under the Mountain until he or his successors saw her debt paid, all Thorin could think was that this she-elf had loved his nephew well. Something, some horrible ache in his chest, eased slightly and Thorin felt his lips curved into something other than a grimace for the first time since… too long. He rose from his throne—no Arkenstone shone above it, instead locked in a chest awaiting its journey to a new, more deserving home—and approached this woman who grieved as well as he for the death of loved ones, and rest a hand on the top of her head.
Everyone in the stone hall seemed to hold their breath, and sucked it in even more when he declared that there was no debt to be settled; she had fought honorably and would be welcome in Erebor as a friend to the Durin’s Folk for as long as he and his kin reigned under the mountain. She had looked up in shock, tears in her startlingly-green eyes, but her voice was steady as she declared herself honored beyond measure and could be counted as an ally to all Dwarves for the rest of her days.
She departed for the Wilds with her white-haired companion—to find the Dunedain and a Ranger by the name of Strider—not long after that public display, promising that if he should ever call for aid, she would answer. Standing before the gates of Erebor, he pressed a kiss to her brow as whispered that she would have made a worthy niece, had fate been kinder. Tauriel’s expression was an amusing mix of shocked embarrassed and pleased, and Thorin held it dear for many years to come. He also held dear the expressions of both elves when he then presented her with a small wooden chest, filled with white gems of pure starlight, placing the necklace around her neck and the circlet upon her brow himself.
The prince’s face when Thorin explicitly stated that they were for Tauriel and Tauriel only was something he treasured most, though, because there was outrage, yes, but it was tinged with respect and no small amount of amusement as well.
After that, Thorin had matters of state to attend to, and did so with help from Balin and, surprisingly, Bilbo for many months before it was time for the Shireling to return to his books and his armchair, to plant his trees and watch them grow. Despite the insistence of all the Company, Bilbo left Erebor with only one small chest filled with silver and gold and—unbeknownst to the Master Burglar until he arrived at Bag-End and plucked up the courage to peek into the box—the shining Arkenstone, swirling with beautiful colors and painful memories.
Of course, Bilbo had then marched straight back to Erebor, to the delight of everyone residing therein, beloved as he had become during his stay, ready to give the Dwarven King a stern talking-to. All thoughts of such things fled when he arrived to see a thin, weary Thorin Oakenshield—with bags under his eyes and grief still weighing down his heart—smile with unexpected happiness at his burglar’s return.
Bilbo had promised to do so, but Thorin could not help but read between the lines, disheartened as he understood that their Hobbit would likely not feel strong enough in fortitude to do so for months, or even years, hence.
The Hobbit never lectured the King Under the Mountain for his trick, though, it did come up eventually and was explained halfheartedly as the only fit solution to the call of gold-sickness, and joked about as Bilbo having claimed it as his share of the treasure anyway. He didn’t leave for many more months after that, but when he did, it was with a Dwarven courting-braid woven into his curls with a beautiful sapphire bead and his poor pony laden down with courting gifts from an overzealous, love-struck King.
They never actually married—not for years and years later—but the Hobbit traveled between the Shire and Erebor every few months with an escort of either Gandalf (rarely) or several members of the Company (most often). After a good long while, he began to be accompanied by his own newly-orphaned nephew, and then eventually Thorin began to make trips of his own to the Shire (decidedly not losing his way those times).
He met his betrothed’s family, and though the terrors that were Peregrin Took and Meriadoc Brandybuck made his heart squeeze in remembrance every time he saw the rascals, Thorin was sure to watch over them especially.
There were good days, and very, very bad days, but Thorin Oakenshield endured for decades as the King Under the Mountain, broken and lacking in his beloved heirs, but eventually gaining a new kind of family that he loved (and protected) with the ferocity of a dragon, though, without the madness that tended to follow.
He eventually ceded the throne to Dain and his heirs, though, not until near the end of his (very long) life.