I've seen a few of these floating around, but one from you of an AU where Steve goes to Asgard and Loki has been a prisoner there since the end of Thor?
He was lost.
Steve had just needed to get away from the noise and chaos of the mead hall for a little while. It was fun, sure, and Thor and his friends were good company, but after several hours of singing and drinking and fighting and shouting and more singing and drinking, he was getting worn out. Not wanting to drag down the high spirits with his flagging energy, or embarrass Thor in any way, he excused himself to take a walk.
But he’d had more Asgardian mead than he thought, or maybe it was just stronger than the earth stuff, because he found himself struggling to remember his way back to the guest wing where Thor had deposited him and Romanoff as Midgard’s visiting warrior-dignitaries. He took a wrong turn, and suddenly, the palace that had been a simple, easily-navigable structure by the light of day transformed into a dimly lit labyrinth.
Which was how Steve came to be near hopelessly lost at night, alone, in the royal palace of an alien world, praying silently that he didn’t accidentally start an interplanetary incident by peeking through the wrong door.
He’d started going down, any time he hit a staircase, hoping he’d eventually end up on the ground level and the palace entry. There were guards there, and one of them would be able to give him directions, he reasoned. Only, he’d gone downward quite a bit now, the air growing cool and dry, no longer scented by the heady flowers that wafted their aroma in from the palace gardens. The corridors were narrow here, and he got the distinct, foreboding sense that he was someplace he shouldn’t be. At one point he doubled back up the spiral stair he’d come down, only to find two bifurcating hallways where he could have sworn there had only been one when he’d come out of it. He took a wild guess at which one had been his original route, only to find the corridor darker and narrower than he remembered. He was on the verge of doubling back again when it opened into a vestibule – a round, stone chamber lit by torches, but otherwise empty. There were no doors, and no windows. And for the life of him, Steve couldn’t figure out what in the hell this room was even for. Were Asgardians just so bored with immortality and overwhelmed with resources that they built rooms with no purpose, just because they could?
Head aching from the mead, and frustrated with his predicament, Steve leaned against the wall with a groan. He let his skull thud pack against the stonework–
–And froze when he heard a click, the stone behind him moving back.
Damnit, was his first thought. I broke a royal palace. No one would notice one stone in one room that didn’t have any purpose, he reasoned frantically, moving away from the wall and looking at the stone that had receded into some kind of depression. It wasn’t cracked though – maybe he could wiggle it back into place?
Steve reached out and touched it, then yelped as the stone lit up, glowing a brilliant green, then a bright gold under his fingers. He drew back, but there was a rumble of stone on stone, gravelly and low, as a whole segment of the wall drew back.
Every bit of common sense was screaming at Steve to turn back, leave it alone, and go find Thor. This wasn’t his world. Wasn’t his business.
But, as Bucky often bemoaned, Steve Rogers had very little common sense.
Pulling one of the torches down from the sconce that held it, Steve made his way into the exposed, narrow corridor, facing the draft of cold air that flowed through it. The stone floor angled downward under his feet, and the hair on the back of his neck prickled. There wasn’t any dust, but he didn’t know if that meant this corridor was often-used and cleaned, or if it had just been sealed too well for too long for any dust to get in.
Mind reeling at all the possibilities, he nearly stumbled when the corridor turned sharply and opened out into a large room.
With another room inside it.
No, not a room. A cell, Steve thought, thinking of the glass cages at SHIELD and the observation rooms that abutted them. Only there was no glass he could see – just a shimmering gold barrier made of energy – or magic.
He stepped closer, curiosity overriding his caution, and felt his breath catch when something – someone – inside the cell moved. A figure on – a cot? A bed? – rolled over, then frowned.
“Is it mealtime already?” a voice murmured, low and flat.
Steve stepped closer still, looking the man over. He was young-looking, though on Asgard that meant little. If he were human, Steve would peg him at about his own physical age. He was pale – from lack of sun, being locked up down here? – and had black hair down back his jaw. He looked puzzled at first at Steve’s appearance, but his puzzlement quickly turned to alarm.
“Who are you?” he demanded, the imperiousness of his voice at odds with the cowed way he scrambled back on the bed until his back was against the wall. “Who sent you?”
“I– no one,” Steve answered, startled. “I– my name is Steve. I got lost.” His frown deepened. “Who are you?”
The man blinked a few times, then laughed. Steve’s shoulders tensed – it wasn’t the kind of laugh that spoke of a well-balanced mind. “I’m no one,” he answered, not looking Steve in the eye. “No one at all.”
“And what are you doing here, No One?” Steve asked carefully. Was this a prisoner? Why wasn’t he with the others in the dungeon that had been part of the tour Thor had offered before?
The man fell silent, staring at a point past Steve, face going slack. “I too, am lost. Have lost.” He chuckled again, going limp against the wall like a puppet with its strings cut: