See, here's the thing about him and Steve: they've always known how to hurt each other.
Tony doesn't mean that metaphorically, or-- or emotionally, but he supposes that's true too. No, he means it literally. Physically.
It had been one of the first things they had done as Avengers. Back when they'd been friends. Back when they'd trusted each other. Back when they could never imagine a time when they wouldn't trust each other. After one too many incidents of mind control, they decided they'd needed... protocols. Contingency plans, for what to do if one of them had been turned against the others. So they'd all sat down and written it all out, the whole team. They'd written out how to take themselves down. How to take each other down.
Thor's basically boiled down to get the goddamn hammer away from him. No one had really been surprised. And mostly they'd just had to hope that they would never need to try it.
(Tony kind of wonders what he'd been thinking when he'd cloned him. He guesses he'll never know now.)
Hank and Jan's files were both the easiest to write and in some ways the most technical. Their files consisted, in the main, of a molecular diagram and manufacturing instructions for a counteragent to the Pym Particles. Hank's own work. There was a second formula for Jan, to make sure the wings and energy blasts were gone. There were complete blueprints for Hank's helmet, and a walkthrough for jamming its ability to broadcast to insects, either by radio interference or physical access to the device. Tony had signed off on that part, and then Steve had done combat evaluations for both of them. Strengths and weaknesses, with emphasis on exploiting weaknesses.
The first half of Steve's file was Hank's work, entirely biochemical; it was a list of drugs known to work on Steve's enhanced physiology, the dosages known to incapacitate him, and suggested delivery systems (mostly dart-based, though gas had potential if you were willing to wait him out until he finally had to breathe, Hank had suggested). Each drug had a projected LD50 for super-soldier metabolism, not that they knew for certain, obviously. And there were notes on how, potentially, to remove or disable the serum, if it came to that. (Tony thinks about it now and laughs.)
Steve had written the second half of his own file, the combat evaluation. Excels at close-quarters fighting, Steve's brisk, no-nonsense notes had said. Yeah, no kidding. The shield can be a weapon, but has primarily defensive uses. Recommended strategy is to engage at a distance, and ideally wait until the shield is used as a thrown weapon. At that point, Captain America will be defenseless until the shield is returned. Projectile and/or beam weapons are recommended.
And Steve had helped Tony write Tony's own file, too. Tony had done the first half, the armor specs. The locations of the emergency release catches for the armor panels. Power consumption and recharge rates on every weapon. Firing patterns. Maximum energy outputs. How to overload and short the armor, given physical access. How to configure an EMP. That half of the file had grown more detailed, as Tony's armor had evolved. After Molecule Man, Tony quietly kept updating the file to include the specs on everything he'd ever done to his body. How to drain or short the battery on his artificial heart. How to induce a heart attack. Preliminary notes on a virus to attack Extremis; he thinks maybe Hank had seen that one, once, when the Skrulls had replaced him. Override codes for the armor, for every model he'd ever had. More override codes, eventually ending in a scrawled note to just ask Cap, he knows them all. Another nanovirus, now designed to attack Bleeding Edge. How to reboot the RT. How to remove the RT. This was, of course, in addition to the AI backup. The one he'd just taken down with ease. So much for that, huh, past self?
Tony never updated his file for the symbiote armor and the newest iteration of Extremis.
He thinks maybe he should be grateful for that now.
Steve had, of course, written Tony's combat evaluation. That part hasn't really changed, over the years. Excels at aerial combat, high-speed attack and evasion, aerial maneuverability, and long-range attacks. Repulsors and unibeam weaponry are extremely accurate at a distance. Recommended strategy, if disabling the armor is impossible, is to close with him and engage in hand-to-hand fighting; the armor still has substantial strength, but speed and maneuverability are significantly reduced. Additionally, use of repulsor rays may be prevented if Iron Man can be induced to fight with closed fists. He is left-handed, and has a tendency to favor that side accordingly. And Steve had been right; Tony's better at picking his foes off from a distance. Oh, he can fight hand-to-hand -- that's pretty much all Steve's doing -- and while he can certainly put a hell of a dent in something (or someone) with the entire power of the suit backing his punches, he's a whole lot slower that way.
When he first made Bleeding Edge, when he first lived with a complete suit in his bones, he used to wake up from his nightmares -- the blurred edges of fights he couldn't quite remember, Steve's face, Steve's face, Steve's face -- in a settling cloud of plaster and drywall, surrounded by new, gaping holes in his bedroom walls or ceiling, gauntlet on his hand, gasping for breath, unable to shake the feeling that the world had gone very wrong.
He wonders if that was how Steve felt when the mindwipe broke.
(For all the things his friends told him about Registration, about the war, there was one thing they never mentioned. Maybe they hadn't known. Tony hadn't known, until he'd watched the footage. Of course he'd watched the footage. In every fight, Tony had closed with Steve. Tony had thrown away every goddamn advantage of having flying weaponized armor. He'd gone immediately to hand-to-hand, where he was weakest, and he'd let Steve hit him, and hit him, and hit him.)
(No one had told him he'd never wanted to win.)
Eight months ago, just before the Cabal took the reins, they'd been an hour out from an incursion they'd thought was their last. Tony had gone to his house in the Hamptons, poured out a row of shots, sat down next to the bar, and aimed a gauntlet at his face. He remembers how bright it was in his eyes, red like the sky of an incursion zone. He hadn't been able to fire.
He thinks maybe he should have.
And now Tony's standing here, at the end of the world, and Steve's coming to kill him.
Honestly, he can't think of a better way to go.
He poured his entire soul into trying to save the world. He blew up an entire alien fleet. He built so very many weapons. In the end, none of it worked. He gets nothing. His friends won't save him. He knows there's no place on that life raft for him. He knows he doesn't deserve one. He'd do it all again just the same. This is his reward: he won't live to see his ultimate failure. Steve will make sure of that.
Steve's standing there in armor Tony made him.
They exchange taunts. Insults. A red flag to a bull.
Steve punches Tony through a wall. They fall into the street below.
Yes, Tony thinks. Just like this. This is what he wants.
The sky is red. The last incursion has started. Another Earth looms above them.
"We are finished!" Steve snarls, and in response, Tony blasts half of Steve's helmet away. He has to be careful. Too much power and he'll take Steve's head off.
He wonders why it hasn't occurred to Steve that he's not really trying.
"I want to hear you say you lied," Steve says, and Tony chokes on his own blood as the shield collides with his throat. He doesn't have a lot of armor left.
Close-quarters fighting. Always his weakness.
Tony spits out several of his own teeth. "I lied," he rasps.
The truth doesn't seem to make Steve any happier. Steve's face is flushed with pure rage, and he's screaming that Tony knew, that Tony knew the world was going to end, that he told no one. Tony tips his head back against the ground. He can't see very well. He focuses on Steve's eyes, pupils dilated with adrenaline, gaze fierce, still that piercing ice-blue. Steve may be serumless now, ninety-five and feeling every year of it, but even in his ancient, lined face, his eyes are the same.
Tony remembers watching Steve open his eyes on that submarine, a decade ago, a lifetime ago. He remembers how happy he was. Captain America was alive.
He remembers that the first thing Steve did was try to fight them.
Well. They've come full circle, haven't they?
"Do-- you know what-- I would have done differently now, Steve...?" he pants. He's still choking, gagging, drowning on his own blood. It's hard to talk. "Nothing."
He smiles, and he's missing half his teeth, and Steve raises the shield. Tony blasts him backwards at half-power. Steve staggers and lunges forward again.
Proximity warning, Extremis whispers. Helicarrier losing altitude. There's a radar plot, with an ID tag for something very large that appears to be falling very rapidly. Right on top of them. Tony's not going to let that be his death. Steve's going to get what he wants, what he's wanted for months, the only thing he wants anymore.
This is his gift to Steve.
Time for the endgame.
Tony pushes the tactical overrides for Steve's suit out through Extremis. He has overrides for every suit he's ever built. Extremis whispers acceptance. Steve has the shield in one hand. In Steve's other hand is a palm repulsor, not that he fights with them; they were for stabilization. But that's easy enough to fix with a few commands.
Weapons override, Tony tells Extremis. He inputs the target. He locks the joints of Steve's suit, wrist and elbow and shoulder.
He smiles again. "And do you-- do you want to know something else?" he breathes, and he gives up his last secret, the one that was never a lie. "I love you. I've always loved you."
Steve's eyes go wide.
Tony remembers Steve's face after Steve had punched him, after Steve had said you used me, after Tony had said and I'd do it again.
He supposes that this is again.
Proximity warning, Extremis repeats, and somewhere above Steve, Tony can just barely make out metal and fire in the bleeding sky--
And Steve, because he always has to have the last fucking word, blusters out a response, another torrent of anger. "What in God's name are you playing at? What the hell am I supposed to do with that?"
How sweet. He thinks it's about him.
Tony laughs. "Supposed to?" he asks, and he chokes out the words on another mouthful of blood. "You're not-- you're not supposed to do anything."
Charge repulsors, Tony tells Extremis, another override. Full power. He locks the rest of the joints in Steve's suit. They're together at the end of the world. There are no more secrets now. No more lies. This is how it should be.
"Steve," he says, and he smiles at him, one last time. "This is a suicide note."
Steve's eyes are wider now, in disbelief, maybe, in fear or despair. Anguish. He doesn't remember the world's ending, Tony realizes. Steve can only think of him. Steve's trying to jerk his hand away and he can't, and what's the matter, Steve, isn't this what you wanted?
Steve's palm is blue-white and bright, bright, bright, with a rising whine of power, bright enough to drown out the red skies and the falling helicarrier, bright enough that the only thing he can see is Steve, struggling against his own armor, desperate and lost, and then even that crumbles and fades away as Tony smiles into the light.