Thor sees it in Loki's eyes, as something breaks utterly. There's no conscious decision, Thor just does it-- he kicks his feet to push his father back, and as Loki's hand comes off Gungnir, Thor is already falling with him.
He doesn't understand what's happened to Loki, but it doesn't matter. He can't leave Loki alone, no matter what's wrong with him. Not when his brother intends to die.
But Thor reaches him, in the void, folds his strong arms around Loki's body protectively. Loki struggles at first, trying to free himself, but Thor holds on. In a moment - an eternity? who knows? - Loki is the only solidity he knows as the unmoored Bifrost yawns forever all around them, threatening both to swallow them up and pull them apart. But they hold on to each other, Loki's grip on his back just as tight as Thor's on his, and Thor lowers his head to shield his eyes in Loki's hair as colors and forces swirl in sickening, incomprehensible patterns.
And he holds on as the cold grows intense, and he holds on as the air burns his lungs, and still it doesn't end. He thinks of Loki falling alone and what might have happened to him without another person there to cling to, and he wonders how long they will last even together.
There is no ending in this, he realizes, they will fall forever between the Realms. Forever, but not alone.
Or so Thor thinks, until Loki's hand comes between them and the void tears open, and all Thor knows after that is pain.
Thor awakens all at once. His muscles, his bones, everything feels strange and weak, not mortal as he had been, but as if he's been pulled and stretched down to his atoms themselves.
There is dark stone beneath him and a crisp star field above him. There is also a person, humanoid, but with a hood and a mask, and it hisses as it talks. "Rise," it orders him.
"You do not command the son of Odin!" he snarls. Defiance is punished as his nerves come alight with agony and he falls flat again, body writhing. It stops.
The creature orders, his tone exactly the same and yet laced with some mockery, "Rise."
Thor gets to his feet, and he follows as bidden. He clenches his jaw, but he also can look around this forbidding dark stone landscape and he doesn't see Loki anywhere.
There are a few stone steps and above that, a floating chair and a large man sitting in it, facing away from Thor. Well accustomed to his own father playing this game of making Thor wait to be acknowledged, Thor grits his teeth and waits.
The figure on the throne speaks in a rumble that sounds like stone tumbling together. "So. A son of Odin. The same Odin son of Bor?" He turns slowly to look down at Thor. He's not Aesir; his skin is greyish and his head too big, but whatever he is, he's bigger than Thor and that makes Thor wary.
"Yes. Who are you?"
"I am Thanos."
Thor knows that name out of tales told in the feast hall. A fearsome foe who battled Bor and Odin in Odin's youth - one of those called Eternals, mortals altered by the Celestials to become immortal. Bor defeated Thanos, but at the cost of his own life. And now someone with Thanos' name has returned. "The name is only a name," Thor says.
Thanos smiles slowly, and the look of it sends a chill down Thor's spine. "Then I will give you proof, Odinson." He lifts a hand and gestures.
Two other humanoids shuffle into view behind Thor, shoving a third between them. That one is a prisoner. His skin glows blue in the starlight and Thor think he must be a Kree at first, he is slender and bare to the waist and the black trousers he wears. His hands are bound behind his back, and he wears a gag and a collar like a hound. It is only when he lifts his head and Thor sees red eyes that he knows the prisoner is Jotunn, not Kree. He steps back in shock and Frost Giant's head drops again, black hair falling down to veil his face. That motion and the look of it is familiar and it's only then that Thor recognizes who is before him. He gasps, "Loki? Loki, what is this?" He rushes forward, knocks one of the aliens aside and grabs for Loki to try to free him. Loki jerks away from him with a muffled cry. "What cruel sport is this?" Thor demands of Thanos. "Free my brother at once of this mockery of foul flesh!"
Thanos laughs. "Your brother? This Jotunn?"
"He's not... he... what are you talking about?" Thor demands, confused. He expects to see Loki's eyes blaze with defiance and anger at this insult, but Loki will not look at him. His eyes stay on the ground at his feet, bare torso hunched inward as if to make himself smaller. "Loki isn't a Frost Giant. That's ... ridiculous. He's my brother. Raised in my house."
"And yet here he is, no Aesir at all. Only magic kept him in that form, Odin's magic hiding the truth. The Frost Gnome had much to say about the lies he was fed before you woke," Thanos says. Thor shakes his head, not understanding how that was possible. He decides it isn't, it can't be. Thanos' tone is mild, but Thor has known Loki all his life and he knows there is a hook being set in these words.
"These are lies!" Thor declares proudly. "My brother is no Frost Giant. Release him and you will face the sons of Odin together!"
Thanos is not impressed. "Oh, but I cannot release him, Odinson, he will claw off his skin. It seems he dislikes the color. Frost Giants are so primitive and easy to upset, are they not?"
Thor ignores Thanos' disdain, and goes to Loki instead. He intends to rip the chains off him, but when he touches the collar, Loki's strange horrible red eyes flare wide and he stiffens, head tilting back as he gives a muffled scream. Thor jerks his hand back, horrified. "What did you do, you monster?"
Thanos grins. "Aesir blood touching him, or his chains, burns in his blood, Odinson. You cannot free him. All you can do is cause him pain."
This is more than Thor can endure, and he explodes, roaring in wordless fury. He wishes for Mjolnir as he rushes the steps to rip Thanos out of his throne and beat him to death with it for this insult. But Thanos catches both of Thor's wrists, and holds him still, as easily as he might a small child. "You are not your father, or your grandfather," his whispers at Thor. "At the least the Frost Giant child grows pure in his hatred. You are a disappointment." He hurls Thor down the steps to the ground.
Thor stares at him, aghast at the strength of him. It will take more than blind rage to take that monster down.
He looks to his minions and seems bored as he orders, "Break the Jotunn runtling."
The chief of them, the one wearing a hood, wields a golden scepter with a shining blue stone in the tip; he presses the tip against Loki's chest and the power flares around him in a halo of lightning, shaking him like a dragon with a sheep in its fangs, until he falls to the ground, still convulsing.
"No, stop! Leave him alone!" Thor rushes him, only to have the scepter leveled at him and he can't move. He stops as if he's run into a wall. He cannot get nearer; he strains, he pushes, he digs for his full strength... and moves scarcely a hand's-width nearer to Loki. He can only watch as the creatures lift Loki up by his wrists, behind his back, so that he hangs in a position that will eventually separate his shoulders. The eyes don't look strange anymore, though they are red; there is a familiar desperation in them. His face, though bluish and strangely marked, is still Loki's, Thor can see that now. He does not seem to have the harsher bone structure of the larger Frost Giants. Nor does any of it matter; wherever this guise arises, it is still Loki within. "LOKI!"
Thanos' laughter is cold and knowing. "Do you want to save him, Odinson? Do you want to stop his torments?"
Now Thor understands this play. He turns slowly and raises his face. "What do you want?"
"Midgard. They have recovered one of my treasures. I want it back. Bring it to me, Son of Odin, son of Bor. Bring me the tesseract, and that... thing you call your brother will be able to rest. Every hour you delay, he will suffer. Bring me the cube and you both may go free."
"How do I know you'll keep your side of the bargain?"
"I suppose you will simply have to... trust me."
Thor glances at Loki, who may look different and Thor may not yet understand why, but none of that matters. This monster is torturing his brother, and Thor needs to stop it before Loki is beyond saving. "Yes, yes, I will," he agrees.
"Then we will discuss the plan. Sit, Odinson."
Loki still hangs there, shoulders bearing his whole weight. "Take him down," Thor insists. "I already agreed to do what you want."
Thanos' smile is merciless. "You misunderstand, Son of Odin. Until you give me the tesseract, he suffers. I will dislocate every bone in his body, roast his Jotunn flesh, and allow my Chitauri to take turns despoiling him until you return. He stays as he is, a decoration for my Sanctuary."
Thor's gaze goes to Loki, horrified, but at least Loki doesn't seem to be aware of the threats, his eyes are closed tightly as he holds himself as still as he can, as his arms are wrenched cruelly up his back. "You are a foul beast, Thanos. And my grandfather should have made sure you could never return."
Thanos is not disturbed by the curse. "He was weak and he died. I did not. I will rule the universe, little Aesir. And there is nothing to stop me." His grin is a thing of teeth and malice. "First you will swear obedience to me. And then you will conquer Midgard and give me my treasure. If you fail to do any of this, the Frost Giant boy will know pain and fear the rest of his days. Do we understand each other?"
Jaw tight, Thor nods once, not trusting his voice, but inwardly promising that he will finish this differently from how Thanos plans. Thor will find help and rescue Loki, and he will destroy this monster who dares to control the princes of Asgard.