This story was entirely inspired by GOLD LUST written by ThornyHedge on AO3 (works #1101546), who is an absolute goddess of fanficdom. That story is written from Fili's pov and I highly recommend reading it first (although hopefully this story could stand alone), along with a 'fix-it' called Desperate Measures written by Finduilas88 which is Kili's pov and can be reached from the link at the bottom of Thorny's story.
We have succeeded. The mountain is retaken and the work has begun to restore Erebor to its once-glorious renown. And the gold, the jewels, the treasure of the mountain is now ours once again. Such were the thoughts that filled the mind of the king these days.
Thorin Oakenshield was now truly King Under the Mountain. His word was law, his every command obeyed without question. But it was not the power of his position that consumed him. He was taken now, as was his father and grandfather, by the Gold Lust, the ache for the riches of the mountain, for more, always more. He could never get enough, would never have enough.
He would stand for hours on end gazing at the endless beauty that was his treasury. He guarded it jealously, covetously and commanded that every piece of treasure found in Erebor was to be brought immediately to the treasury on pain of death, should any be stolen.
Surely, his people thought, surely their king meant to restore the treasure to them all. After all, it was they, their ancestors that had pulled it from the mountain. They had waited so very long, worked, starved, bled and died for him. Surely he would reward them when the last of their people had returned to the mountain. But such was not to be as daily they watched him slip away, lost to them all, save one, the crowned prince Fili.
"What is it your brother always called you, Fili…kidhuzel?" Thorin asked in one of his more lucid moments as he and Fili visited the treasury again. His left hand was around Fili's shoulder, stroking gentle but possessive circles there. His right hand palmed his staff in his breeches, already achingly hard. "It fits you, gold-of-all-gold," he continued, for truly Fili was his most longed for treasure. He looked upon the golden prince and did not see the tears that glistened in Fili's eyes, for they were to him shining, colorless as the jewels so desired by the elves. He did not see Fili's hands gripped so tightly that little half-moon marks of blood remained when he loosened his fingers. Thorin only saw what he wanted to see these days, what the gold-lust allowed him to see. And he saw in Fili a treasure, a receptacle for his seed, a consort.
"You are the most glorious treasure in Erebor, my precious Fili," Thorin breathed as he rubbed himself against his nephew's rear. It never even registered in Thorin's gold-addled brain that the prince was not erect, not in the least bit aroused by him. To Thorin, Fili ached for his king as much as Thorin ached for his prince.
But Fili was full of sorrow for his uncle and fear for himself. He knew what was to happen next, what always happened. It was the same, day-in and day-out when his uncle forced him to visit the treasury. Thorin would divest Fili of his breeches and push him down onto the hillocks of golden coins, goblets, trinkets and troves. The gems would cut and bite into his legs and knees as his uncle mounted him, pressing into him, not gently as his Kili would love him. 'Oh, Kili,' Fili groaned to himself. 'My azyungal, my sanadad. I will never be clean enough for you again. Forgive me, forgive me, my beloved,' he sobbed openly now, remembering how he'd had to shame himself and betray his One by removing Kili's braids and beads from his hair, only to be forced to replace them with Thorin's.
Thorin was so filled with lust and craving that he never even remembered to prepare Fili before entering him. Fili's cries served only to push Thorin's lusts higher, for to him those were the cries of a satisfied lover being undone by him, by his treasure. "Yes, my golden one, MY kidhuzel. Scream for me!" Thorin would shout as he grabbed Fili's long, silken golden hair. Fili would tear and bleed, the thick, copious blood at last giving some relief from the friction of Thorin's endless rutting, even as it coated Thorin's relentless staff. Tears would fall down Fili's face as he thought of all the times he had ached for Thorin's notice, had coveted the affection his uncle would so easily show to Kili.
Thorin had always found it easier to show affection to Kili. The younger prince was dark of hair and eyes, quick with a smile and tried so hard to win his uncle's favor. It was not Kili that would succeed Thorin to the throne, so the king could be more relaxed in his treatment of the boy. And it was not Kili who reminded Thorin daily of the gold beneath the mountain that had claimed his father and grandfather so long ago.
Not since Thorin's own brother Frerin, had there been a more beautiful and perfect specimen of dwarfdom than Fili. Thorin gazed at the prince now, draped in gold and jewels and thought back to the lust that had always eaten away at him. Thorin had tried to keep his distance from Fili, allowing Dwalin and Balin to tend to Fili's training and learning. The one thing that Thorin himself had schooled Fili in had nearly given the king reason to ravage him years ago…the forge.
He had taken Fili with him to the forge to teach him the craft of working the metal. Before the day was out, Fili was shirtless and glistening with sweat from the heat of the fire and Thorin was achingly hard with stains of pre-release hidden thankfully by his tunic. Thorin watched each day as Fili's muscles swelled and grew from handling the smithing hammer, his shoulders grew broad with the exertion. He fought every day of Fili's youth to resist the call of the gold, but in Fili, he saw his undoing.
Many were the nights that lust for the young golden prince dragged Thorin down into its clutches. Many were the nights that Thorin stroked himself to a sorrowful, shameful release with Fili's name like a prayer on his lips. And many were the nights on the quest that Thorin would not sleep. Instead, he lay awake listening to the sound of his nephew's coupling, their love so evident daily in both word and deed.
It was that love now that gold-lust twisted and told Thorin was his own, that Fili no longer belonged to any save himself. Each day Thorin's desire for the prince grew, and each night Thorin castigated himself for that desire, even as he gave into the lust that sought to overtake him. He had been able to hold it at bay until Erebor had been reclaimed and his visits to the treasury began. It was there that the gold-lust claimed him, appealed to him, whispered to him, told him that every sick and twisted desire of his soul was his for the taking. And it was there, at last, with Fili by his side, that Thorin, King Under the Mountain, yielded and began to listen to the call of his own, his One, his golden treasure.