From a kink meme prompt:
During Thorin's recovery after the Battle of Five Armies no one has seen the hobbit and everyone assumes he's long gone. Which isn't what actually happened.
Somehow, the Bilbo managed to get lost in the corridors of Erebor and (probably by a blow to the head) lost his memory. Not knowing where or what he is, only that he's small and cold, and doesn't like loud noises, he hides in the farthest walkaways of the kingdom, stealing food whenever he can and hiding from the dwarves who are big and loud and scary to him now.
After a while (few months? a year?) some dwarves start telling stories about a ghost living in the abandoned parts of Erebor - of the sounds of ghostly footsteps and missing food, etc.
Thinking it's an estranged ork or some thief, king's nephews - freshly recovered and looking for an adventure - decide to check the rumors out. And they find the burglar, oh they do.
Crossposted on AO3 as well. I am not an expert on all things Tolkien so if there are mistakes and errors, forgive me. This deviates from canon anyway so just go with the alternate universe thing. Feedback is loved.
In the months after the reclaiming of Erebor and the Battle of the Five Armies, many dwarves who had once fled Erebor returned, seeking a place to call home once again. They brought their families, many of whom had never set eyes on the mountain before. Among them were other dwarves from Dain's kingdom, seeking to help their brethren. With the ever growing population, the restoration of the great dwarven kingdom was starting to get well underway. Everyone did their part, even those who were still healing from the battle.
"Laddie, if I have to tell you one more time to take it easy, I'm going to get Dwalin to chain you to the bed," Balin admonished somewhat fondly. It was the fifth time that week that he had caught Thorin working when he should have been resting.
"I am well enough for paperwork," Thorin insisted, though he couldn't hide the slight wince when he turned to face his old friend.
"Leaning over that table isn't going to be helping your ribs heal, nor your shoulder," Balin said pointedly.
Thorin scowled and his uninjured hand went to his broken shoulder automatically. Most of his other injuries had healed but because he kept pushing himself, he had managed to slow the healing process on his more severe wounds. "I'm fine," he insisted.
"Which is why you're not writing with your dominant hand," remarked the older dwarf dryly.
Thorin threw his quill down on the table. It was true that he was having to rely more on his other hand at the moment, slowly writing shaky letters and approving requests for but that did not mean he was not well enough to work. Erebor needed him, after all and he had put too much into reclaiming his kingdom to stop now.
"The kingdom isn't going to stop just because its king needs some time to recover from injuries he gained in the war to defend said kingdom," Balin said. "Now, get away from that desk and come join the rest of us for dinner."
Balin always insisted that Thorin join the rest of the company for at least one meal a day. If not, he feared his king would simply work himself into an early grave. Guilt weighed heavily on his heart, Balin knew, and Thorin threw himself into ruling the kingdom to distract himself, as though it would somehow make up for the wrongs he had committed.
Stiffly, Thorin rose and joined Balin at the door. He had been sitting at the desk for too long and he knew Balin could tell. The expected reprimand did not come however, as it had so many times before.
"You need to take more care, Thorin,"Balin said softly.
"There will be time enough for that when Erebor is prosperous once again," Thorin replied.
He and Balin walked together to the hall where the others were waiting. It was part of the royal chambers, once used only by the royal family in the old days and now used by the Company to gather and catch up at the end of the day. At other times, they would use the huge communal halls where all the other dwarves would gather but their evening meal was just for the thirteen who took back Erebor.
'But it should be fourteen,' Thorin reminded himself. There was one who would always be missing from their private gatherings, one who had come far from home to help them, who had tried his best and then been cast aside for his efforts.
Bilbo Baggins had not been seen after the Battle of Five Armies. Some said his voice had been heard, shouting the arrival of the eagles but he had not been seen since the battle truly began. Thorin and the others all dearly hoped he had returned to the Shire safely, as he had not been counted among the dead, but there had been no time for them to check. Gandalf had disappeared shortly after the battle and Thorin hoped that the wizard would return with news of the hobbit soon. If he could only just discover the fate of the halfling, then perhaps he could begin to make amends.
"Thorin." Balin stopped him just before they reached the dining hall. "It would not do to let the others see you looking down. They will worry."
"It is not me they should worry about," Thorin muttered.
"Aye, we all worry about him as well, but there's nothing we can do for him until we learn what's become of him," Balin said, trying to be practical. "We'll decide what to do once we have news. Now, buck up. They'll want to know their king is healing, not pushing himself to work so his recovery time is doubled."
Thorin nodded and did his best to put thoughts of Bilbo out of his mind. There were other concerns that he would be better off devoting his time to, like the exploration of the long-abandoned corridors or Erebor and finding living spaces for the dwarves that were arriving daily. He received official updates and reports from various appointed officials, dwarves who had been part of Dain's court and had stayed to cement an alliance between the Lonely Mountain and the Iron Hills. Thorin did not know them well and was not altogether sure if they could be trusted so he often relied upon conversations with his Company to get a more accurate sense of how things were in the mountain.
Today, it seemed as though the restoration was progressing fairly smoothly in Erebor. A cave in had been discovered in one of the lower mines but it had been cleared without incident and the tunnel reinforced. Exploration of the upper levels was also going well.
"Yes… for the most part," Nori said. "There's a minor hiccup in getting people to venture into the northward deep corridors but I'm sure that will all get sorted out soon enough."
"What kind of hiccup?" asked Thorin, narrowing his eyes.
"Oh, nothing big. Just… the last team we sent down that way came haring back with tales of ghosts," the dwarf replied, shrugging. "It's probably nothing."
"Ghosts.." Thorin sighed skeptically. If the mountain was haunted, he was sure he would have learned as much when they had first arrived.
"Aye. They hear running footsteps and other noises but there's no sign of anyone actually being down there. There shouldn't be anyway. We haven't been able to finish surveying it," Bofur adds.
"They're not the only ones talking of ghosts," Bombur put in. "My kitchen assistants have reported strange noises and food going missing but they can't find any trace of anyone sneaking around."
"And does no one think to question why the dead need food?" asked Thorin. "It is far more likely that we have a thief in our midst."
Dwalin snorted. "If they're blaming ghosts, I don't think they're going to be thinking too rationally about the whole business."
"I've already asked about if anyone has any information," Nori said. "If there's anything to know, I'll find out in a few days."
One of the first things the sneaky dwarf had accomplished after dwarves had begun arriving at Erebor had been to set up a network of informants. Dori had not been particularly happy about it, but as his brother insisted on using his talents of spying and thievery for the good of the kingdom, he did not actively prevent it. Thorin endorsed the venture, trusting Nori and knowing that such a network would prove useful.
"Let me know what you find," Thorin said.
After that, the conversation turned to other things. The company chatted among themselves, sharing stories and news of Erebor's restoration alike. Eventually, even that died away into yawns and sleepy silences and gradually, they bade their friends goodnight.
Fili and Kili were among the first to retire, still unused to much activity after their forced bed rest. The brothers had both been grievously injured in the battle to the point where no one had been sure they would survive. Through some miracle, both had pulled through but their road to a full recovery had been slow. Neither of them had liked being confined to bed rest for weeks, first in the healer's tents and then in the royal suite inside Erebor. All the brothers had wanted to do was to explore the mountain and do their part in rebuilding the kingdom but so far, they had both been denied their wishes. Mostly. Thorin had offered to allow Fili to aid him dealing with the reports and dealing with advisers - the less strenuous parts of his duties as king. Fili had turned him down. Bored though he was, he was not that bored, as he informed his uncle.
"You will have to accept responsibilities like these in time, Fili," Thorin had informed him. "You are my heir."
"And I will," Fili promised. "But perhaps when Kili won't be an unholy terror over being left to recover in our rooms alone."
But that had been a couple of weeks before and Thorin had not pressed for them to do their part for Erebor yet. There was no doubt in the princes' minds that when he did finally realise his nephews were well enough to aid him, they would be called upon for the most boring of tasks. Neither Fili nor Kili had any interest in reading dry reports about food supplies and and such things, not when there was still a mountain kingdom to explore.
"So what do you think it is?" Kili asked, once they were safely away from potential eavesdroppers.
"Who knows?" Fili replied with a grin. "A stray orc, perhaps? Something that got in when Smaug still lived? Or maybe it really is a ghost."
"And as princes of Erebor, surely it is out duty to ensure that there are no threats to our newly reclaimed home," Kili continued, matching his brother's expression.
"Of course. Any orcs should be routed as soon as possible. Any thieves should be caught. Any ghosts should be put to rest," added Fili. "And since no one else seems willing to go down there, then obviously we should be the ones to do it. Leading by example."
"Exactly." Kili clapped his brother on the shoulder. "They can't argue with that."
"But they'll try, so we should be up early in the morning to avoid the crowds," Fili replied, returning the gesture. "Sleep well, brother."
Both brothers woke early the next morning. Fili was up first, prodding and poking his brother until he rose too.
"If we don't hurry, there will be too many people between us and the northward passages."
Kili mumbled and rolled over, all but falling off the bed. "I'm up, I'm up. Let's go…"
"Put your trousers on first, brother," advised Fili.
Finally, dressed and armed, the brothers set off for the kitchen. They would need breakfast before they started their investigation and something to take with them for later, if their exploration took a while. Kili, being lighter on his feet than Fili, was the one to sneak into the kitchen to grab whatever he could. There were a few sleepy kitchen assistants around but they were busy enough not to notice or think to ask what he was doing.
With food and water for the day, the brothers set off for the unexplored northern passages. Behind them, Erebor was waking up but ahead of them lay only darkness and quiet.
Once, the northward passages had been home to miners and their families. The lower tunnels led to the mines where they had worked. Now, abandoned for so long, no one was sure if the tunnels were still structurally sound. The investigation teams had only been so far into the area before turning back. Chalk marks on the walls showed their progress, coded notes for the restoration teams that would be sent to make the area liveable again.
"It looks like this was as far as they got," Fili said, holding his torch up to a set of marks on the wall written in a shaky hand. "'Noises in the deep. Will go no further.' But it doesn't say what kind of noises."
"We'll just have to find out ourselves," Kili said. "Shall we split up to cover more ground?"
Fili considered for a moment. "As long as we do not go too far into the tunnels," he replied. "Perhaps we should investigate the dwellings on this level? I don't think we'll find much if we just stay on the paths."
Kili lit his own torch from his brother's and they split up to investigate the abandoned homes on either side of the passage. They were small homes, used mostly by single dwarves but there was evidence of families - abandoned children's toys, a table set for a meal for two that never happened. It filled Kili with a deep sense of sadness. People had lived here once, but there was nothing he could see that would tell him who they were and what had become of them Had they escaped when the dragon had come or had they died? Kili stood in the entrance of a small home, looking around at upturned furniture and debris.
A faint noise brought Kili abruptly out of his musings. Fili was in a home across the other side of the tunnel still. Kili would have noticed if he had come closer.
The sound came again, a quiet shuffling in another room. Kili rested his hand on the hilt of his sword and edged deeper into the room. There were doors leading off from the main room and Kili carefully pushed open each one. They creaked horrendously loudly in the quiet, hinges long rusted.
The first room turned out to be a bathroom. Seeing no sign of life, dwarf, orc or otherwise, the prince turned away. The next room was a bedroom, also empty - or so Kili thought. A soft sound caught Kili's attention as he turned away. It sounded like… a gasp? He turned back, looking for whoever had made the noise. As he turned, there was the same quiet shuffling from the corner behind him. He turned, raising his torch. There was no mistaking it that time - a scared whimper.
"Hello? Who's there? I promise, if you don't hurt me then I won't hurt you," he said quietly. He laid his sword down on the ground as proof. "Will you come out?"
"You won't… hurt me..?" The voice was whisper quiet, hoarse from misuse.
"I promise. Neither me nor my brother will harm you," Kili repeated.
He watched with wide eyes and as a figure emerged from the shadows. Though he was pale and dreadfully emaciated, there was no mistaking those wide eyes and hairy feet. Kili nearly dropped his torch in shock.