They decide to put Loki in Thor’s corner of the infirmary and leave the device down in containment - not because they think it will keep adult-Loki from getting stronger, particularly, but because it’s logistically easier. Loki assures them that physical proximity is no longer necessary for the spell to work. There is even an observation window overlooking the infirmary, so the Avengers (and Coulson) can watch from a safe distance.
It is all logically and calmly decided, right up until Loki can’t make himself go into Thor’s room.
“I - I will do it from here,” he tells them brightly, and then can’t get the shimmering thing to materialise.
Clint goes out to meet him in the hallway. “Will it mess you up if I go in with you?” he asks. Steve is glaring at him from the observation window, but Loki’s miserable expression eases a bit.
“No. You will be in no danger and my concentration will probably be better,” he admits.
Neither of them pretends it’s strictly necessary for Clint to be holding Loki while he does the spell, but it does get him up high enough that he can see Thor clearly and it puts Clint in a good position to physically restrain him if adult-Loki gets frisky. Clint can practically feel Steve’s glare on the side of his head, but nobody actually says anything.
“All right,” Loki says, voice trembling. “All right. I am ready to begin.”
He closes his eyes. The shimmer begins to grow around him, rippling over Clint’s skin with a surprisingly pleasant tingling feeling.
“Clint?” Steve asks over the intercom.
“We’re fine,” Clint says. “It tickles a little.”
The shimmer spreads over Thor, who twitches slightly. Clint closes his eyes, knowing the white flash is coming next, and for a moment all he can see is the redness of his own eyelids as the flare of light washes over them.
He opens his eyes. On the bed, Thor is opening his.
“Thor?” Steve says.
Thor blinks and looks around. His gaze falls on Clint and Loki and he looks confused for a moment. Loki’s hand tightens nervously on Clint’s collar.
Thor’s whole face lights up. “Brother!” he says, overjoyed.
And that’s when adult-Loki loses his shit.
Clint barely keeps a hold of him as he arches his back and tries to lunge at Thor, screaming and spitting in what must be some dialect of Asgardian not meant for mortal ears - either that, or it’s just gibberish. Clint manages to drag Loki back from the bed and get down to his knees so that if Loki breaks free he won’t hurt himself falling, and then he just tries to hang on.
In the background he can hear Thor demanding answers and everyone in the observation booth running down the hall. In his arms Loki has mostly gone silent, saving his strength for escaping Clint’s hold. He writhes and twists, clawing at Clint’s face and arms.
“Loki!” Clint yells. “Both of you! You’re too weak after that spell to do anything so calm the fuck down!”
Loki gives one last vindictive heave in Clint’s arms and goes still, panting harshly and vibrating with tension. Out of the corner of his eye, Clint sees Thor do that thing Clint is occasionally so jealous of where he grabs Mjolnir’s handle and is suddenly wearing full armor.
“What is the explanation for this?” Thor demands. “Why is my brother a child and what is wrong with him?”
“Thor!” Finally, Steve’s in the room. “It’s a long story, but there was a magical accident, you were in a coma for a while, and now the child and adult versions of your brother’s personality are fighting for control of his body.”
“I see,” Thor says, taking this in stride, and Clint takes a moment to thank god that Asgardians are used to dealing with freaky shit. He steps towards Clint and Loki. “Brother - “
“Get that look off your face, Thor,” Loki snarls. “I want none of y-your p-p-” he makes a strange choking noise. “I am sorry, I am so sorry, I did not want to hurt anyone, Thor - “
“Shh,” Clint says. “We know. It’s all right.”
“Brother, be at ease,” Thor says, moderating his usual booming voice to a low soothing rumble. “We are none of us injured and you have done nothing wrong.” He reaches out, slowly, so Loki can see him coming and won’t be startled. “Do not worry - “
Loki sinks his teeth into Thor’s hand. Thor freezes.
“Brother, you cannot talk if you have my hand in your mouth,” Thor reminds him calmly.
The irritated look on Loki’s face as he opens his mouth and lets Thor’s hand go would be comical under other circumstances.
“Did he perform the spell to awaken me?”
“Yeah, about a minute ago,” Clint says.
Thor nods and regards his brother seriously. “Loki, you must replenish your energy after such exertion. You know as well as I that it will not benefit you to be recalcitrant on this point. If we go up to the kitchen, will you take sustenance?”
Loki shivers. “Yes, please. Thank you, brother.”
Thor smiles tenderly at him, and then lifts Clint to his feet without any apparent effort.