It's as though he's been thrown through the air, engulfed into a lake of pure, liquid fire. His mind conjures up the thought of the sun, or perhaps molten lava: he is awake, clear-minded. Armin is conscious of his transformation, and he is conscious that he must stifle the urge to panic. He's hyperventilating as his synapses kick back to life, air rushing to his lungs.
Miraculously, he's not in any pain. Perhaps the injection has dulled his ability to feel anything. Armin knows he must keep his goal in mind above all else, even though he wants to tear his way out of this new form that explodes around his frail body.
But he wants to live, to see the world and the faces of his friends again—Eren and Mikasa and Jean, he thinks, and with the thought comes a fierce and irrefutable burst of joy, tempering his resolve—and so he rises above Shiganshina's rooftops and houses, no longer as a boy but a Titan.
Bertholdt lies upon a hot roof, withered and limbless and trapped. He seems so insignificant, a tiny and pithy scream torn from his lungs. He calls out for his friends, then Reiner and Annie. Armin does not fault him, but he feels nothing for him, either. Now Bertholdt is squirming under his jaws, and his scream soon breaks as easily as his body.
So it is done. Armin falls, or rather, his Titan crumples. Steam erupts from the frame as it dissolves.
He shakes and shakes and shakes as they drag his half-naked body from the burning carcass and into light. He's burning through his clothes and they are all shouting, but he cannot discern words. Eren and Mikasa and Connie and Jean are there first, their faces swimming in and out of focus.
How many of us have died? What happened to the Beast Titan? What about the Commander?
He wants to articulate this, but his nose is bleeding so badly it's all he can taste. His breath is ragged as he looks at them all, the colours suddenly too bright.
It doesn't matter. You're all alive, he thinks. We've won, if only for the moment.
And Armin smiles vaguely through the dark blood oozing into his mouth, staining his teeth and skin, and at last he faints.
a/n: Realistically, I doubt Armin's Titan transformation would go as smoothly as it did in the comic. That's not to say he'll die, per se…although I like to think it ends ambiguously, I also think Armin will be okay with a ton of rest and probably more Titan serum or something.