Based - with kind permission - on an idea by the great mistress of angst and whump, CurtisMcQueen8
I messed around with the timeline of the movies a bit because—because I can.
Shuri manages to analyse the mind stone's memory. When she finds a way to turn the data into images, she comes across a familiar face. She freezes the footage to try and remember who he is. She's seen him before, not face to face though. And then it hits her. It's hard to recognise the man who invaded Earth from this image, but it's him. It's Loki, Thor's brother, and he looks like a man in hell. A man who has been in hell for a long time. A man who, as far as she knows, has faced Asgardian justice for trying to take over a planet. What happened to him? When did it happen, and where? She's still staring at his face instead of trying to work out how to maneuver the massive amount of data stored in the mind stone. He looks starved, his face is covered in cuts and bruises, and there's so much blood. Shuri pulls herself together and gets to work. When she finally shuts off the stone's energy source she's exhausted and feeling sick. She needs to speak to Thor as soon as possible.
Thor is staring at Shuri's screen, eyes narrowed, fists clenched. He hasn't said a single word since she started the footage. She doesn't know what he's seeing, doesn't want to.
"Thor? Do you want me to shut it off?"
"No." Thor's voice is flat. Thunder has been rolling outside for a while, Shuri realises only now.
"He's lived through it. I can at least sit through watching it."
"What's—can you tell me…?"
"No! He'd kill me with his bare hands if I let you see this. Any more than you already have."
Thor stubbornly refuses to shed tears while he's watching, while Shuri is watching him. But he's horrified. Pure bloodlust is boiling in his veins and he's coming up with more and more brutal ways to make Thanos suffer, but underneath he's paralysed by the pain and the terror in his brother's eyes. The breaking bones. The gashing wounds. The hunger. The dehydration. The humiliation. The tears. The mind fucks he can only imagine based on Loki's panic and despair. Shuri has given him headphones, so she can't hear what Thanos is saying, can't hear Loki's replies, and as the mind stone mercilessly keeps showing the events, she can't hear Loki's silver voice break and his eloquence shatter to screams. Nor can she hear his screams being reduced to sobs. On the screen Loki's abused, bloody body is sagging, held up by the chains around his wrists. His brother's eyes are empty and he's gone completely silent. But he's conscious. How can he still be conscious.
Finally it's over. Thor allows himself to close his eyes for a moment, still not crying, still raging, furious, and then he realises he should have known. He should have known his brother had been broken and used against his will. How could he not see? How could he fail to see how much his brother had suffered? Instead he had hauled him back to Asgard in chains, to be brought before the all-father like a war criminal. And when Odin sentenced him to eternal imprisonment, almost as cruel as his original death sentence or maybe even crueler, Thor had done nothing. He hadn't gone to see Loki in his cell. He had never asked what had happened to him after he'd fallen from the rainbow bridge. Everybody had just assumed the younger prince had thrown a tantrum and was either mad or evil or both, and Thor couldn't be bothered to question any of it. And now his younger brother was locked up in the deepest dungeons of Asgard, to be ignored and forgotten, punished for a crime he was the victim of, not the culprit. And that's when Thor finally does cry, his heart breaking for the proud man his brother used to be. Still is, refusing to defend himself because it would mean admitting a weakness.
Thor straightens and wipes at his eyes.
"Thank you for telling me, Shuri. I must return to Asgard immediately. My brother deserves justice."
Odin has been listening to Thor without interrupting him once. Now that his son is finished he just asks:
"Why did he never speak about this?"
Thor heaves a sigh. His voice is full of guilt when he says:
"He probably thought we wouldn't believe him. And, to be fair, we probably wouldn't have. Without evidence."
Loki refuses to look up from his book when he hears his brother's footsteps approach his cell. After all this time… how much time exactly? It's hard to keep track down here, and what's the point anyway. Still not making eye contact Loki spits venom at Thor:
"Precious prince. What an honour. Have you come to mock? To gloat?"
Thor stops in front of the cell, and then he deactivates the force field. Loki narrows his eyes, confused and irritated, but doesn't comment or change his posture. Thor steps inside the cell.
"I have come for forgiveness, if you can find it in your heart. We have failed you. I have failed you, brother."
Loki puts down his book and stands, just as tall as Thor. He stands very close, and a wave of different emotions washes over his face. He's waiting. Thor puts his hand on Loki's shoulder, prepared for his brother to shrug it off or slap it away, but Loki just waits.
"I know what happened, Loki. After your fall from the rainbow bridge. They have found a way to access the mind stone, that's how I know what Thanos—that it wasn't you. That he forced you to do what you did on Midgard."
Loki goes even paler than he usually is. Thor gives his shoulder a squeeze.
"You're free, brother. I've come to bring you freedom, and hopefully, some day, justice."
Loki doesn't say anything. He just sits back down in his armchair and reaches for his book, his mind racing. Thor looks confused for a moment, but then he sits down on the floor of the cell, his back against the wall, and reactivates the force field. Non transparent this time. Loki ignores him. Thor takes a breath.
"So you were innocent. All this time you spent down here you were innocent, and yet you just accepted Odin's sentence and never even mentioned any of it."
Loki drops the book. His eyes are empty and his voice is flat.
"As if Odin would have believed me. As if any of you would have believed me. Except Mother, maybe, but I couldn't..."
Loki closes his eyes at the idea of telling his mother what happened to him. Thor nods slowly.
"You're right. I wouldn't have believed you."
Loki looks at Thor, surprised by his honesty. Thor goes on:
"And you know why? Because until the moment Shuri showed me the footage I'd thought nobody could make my brother do anything he doesn't want to do. I didn't think it was possible to break your will, Loki. You're the most stubborn, proudest, strongest person I know. It was unthinkable that anyone could overpower you. So yes, it was easier to believe that you were evil than to believe anyone might have forced you. I'm sorry, Loki. I should have asked. I should have listened. I should have known that wasn't really you. I should have stood up to father and to the authorities of Midgard. I should have achieved justice for my brother so much sooner. I'm sorry."
"How much did you see?"
Thor knows what he means without explanation.
"Everything."
Loki looks away, his eyes darting across the small space of the cell as if looking for a place to hide.
"I'm sorry, brother," Thor says as gently as he can. "I had to, I owed it to you."
"How did it feel?"
"Watching the footage?"
"Yes. How long did it take for you to be disgusted with my weakness?"
"Loki, were you listening to me earlier? You're not weak! You resisted torture of the worst..."
"Only I didn't!" Loki yells, jumping up and towering over his brother. "I surrendered! I did as I was told! I obeyed!"
"After how much time? How much pain? Loki, he had the mind stone. Anyone else he could have just controlled. You are so much stronger that he had to break you first. And I'm so sorry you were alone in that. I should have come for you. I should have found you. I should have helped you fight him. I can't even imagine what it was like."
"Infinite."
Thor looks at his brother quizzically. Loki goes on without looking at him, talking to himself more than to Thor:
"His is a dimension without time. So anything that happened, anything I felt, was eternal. It would never end. That's what finally broke me. He wouldn't let me die, and I couldn't even rely on time to finally end my existence. I don't remember how it all ended. I barely remember New York. For the first weeks here in the dungeons I didn't even remember why I was a prisoner."
Thor closes his eyes in pain, leaning the back of his head against the wall.
"I'm sorry," he keeps whispering, and eventually he gets up. When he pulls Loki into his arms, his brother doesn't even try to avoid the embrace.
"I'm sorry I didn't come for you," Thor says once more.
"You did eventually," Loki answers him, and there's the hint of a smile in his voice when he finally allows himself to relax.
Odin looks straight at Loki.
"Loki Odinson, Prince of Asgard."
At the title, Loki looks at his mother, surprised. She smiles and inclines her head.
"The realm of Asgard, and with it all the realms, including Midgard, revoke your sentence. Your name is cleared. You're a free man."
Loki takes a breath and nods at Odin, waiting for the all-father to add a personal comment. Frigga is giving her husband a look that both her sons can read as clearly as a written note:
'A true king admits his mistakes.'
When Odin stays silent she steps forward.
"My son," she says gently. "I am so sorry for what you had to suffer, both at your enemy's hands and by our own, flawed justice."
Loki swallows at the mention of Thanos but forces a smile. It turns genuine when Frigga opens her arms and pulls him close.
"I am so glad it's over, Loki."
'It will never be over,' Loki thinks, 'not for me.' But he kisses the top of her head and holds her against his chest for as long as he dares, in public, while Odin is waiting.