Gate to seal it. Roy doesn't let him make that sacrifice alone.
The idea for this came from a prompt by an anonymous poster at the FMA Kink Meme. Thanks OP!
A flash of golden hair followed his turn, and he was gone, vanished into the darkness of the airship, Edward Elric offering himself up for sacrifice yet again. And Roy Mustang stood on the slowly separating platform, once again watching, once again letting him go. He should have known it was too good to be true. The Fullmetal Alchemist was not the type to hold still for long, and it simply wasn't meant to be. At least now he knew the young man was alive, and that had to count for something, didn't it? Maybe simply knowing that the young man was out there somewhere, just out of reach, would be enough for Roy's world to rock back into some kind of alignment. The dark haired man turned his single dark eye on the distraught but now quiet youth beside him. Clapping a hand on young Alphonse' tense shoulder, Roy knew what he had to encourage the boy to do.
The General had thought he was dreaming when he'd first seen that glowing golden hair, that too familiar face and form crouched beside Alphonse on the spear of rock arrowing towards the enemy aircraft, but this wasn't like any of the dreams he had before. It was the strange, subdued attire that helped Roy shake off his disbelief, convinced him that this was really happening. He didn't believe in a destiny pre—programmed into the pattern of life, but marvelled at the coincidence of his being in Central at the same time as the otherworldly invasion unfolded. And marvelled as well to find Edward, after two long years secretly mourning his loss, a bit older, more centred, just as bold and still so uniquely beautiful, right in the thick of things, just like the good old days. Hope long since abandoned blossomed again. Hope for friendship, with the chance for something more. Hope that was now once again dying.
When Hawkeye told him Alphonse had been discovered, literally in the flesh, alone in the city hidden under Central with no sign of Edward, Roy had levelled a glare at the ceiling above his hospital bed and flatly refused to believe that Alphonse alive meant Edward was dead, equivalent exchange be damned. It was inconceivable that the young alchemist, so relentlessly determined, so passionately alive, could simply be . . . gone. But as the weeks turned into months and finally into years, and no word was heard, finally only Alphonse truly held any real hope of ever seeing the Fullmetal Alchemist again, and Roy had resigned himself to regret not confessing his feelings to his youngest subordinate while he'd had the chance. Even in the thick of the endgame, watching the young man running to his little brother's rescue, Mustang had thought there would be time later, when the battles were won and the goals achieved, to find out if his suspicions were true, if the heat in the boys scorching amber glare was fuelled by more than just anger. But then, suddenly, unbelievably, it had been too late. Edward was lost, and Roy was left frozen without the boy's blazing fire to warm him. And now, it was happening again. Deja vu.
"What are you waiting for, Alphonse?" Mustang said quietly as he gave the youngster a slight nudge towards the idling airship. "You've spent all this time searching for your brother. Are you going to just let him disappear from your life all over again? Go on. You don't have much time."
"As much as I'd like to, Sir, I can't," Alphonse said, deep regret in his tone. "I have to close the Gate on this side. I'm probably the only one in Amestris who can actually do it, and it's my responsibility." The young man turned solemn eyes sparkling with unshed tears on the older man. "So you should go, Sir. Before it's too late. Brother deserves better than to always walk alone. And you deserve better than to quietly freeze in the dark, regretting things left untested."
Mustang froze, jaw dropping, and Alphonse made an impatient sound, hand on the small of the older man's back urging him towards the ever widening gap. Roy's eye locked to Alphonse face, shocked at his suggestion. Go with Edward? "Alphonse, what . . . I don't. . . How did you . . ." The airship canted slightly, and the gap was suddenly much wider. Mustang stood, still frozen, teetering on the edge of uncertainty.
"We don't have time to discuss this! In a moment you'll have missed your chance!" Alphonse sudden anguished shout spurred the General into motion, decision made.
Mustang ran full speed toward the gap between him and the other worldly aircraft. The flying machine shuddered as he approached, tilting slightly as Roy sprang to clear the gap, arms outstretched, hoping his limited depth perception didn't play him foul. For a moment he thought he wouldn't make it, and faced a very long fall to sudden death at the bottom of a scream, but Roy just managed to span the distance. He landed hard on his stomach, legs stretched out over thin air, and the plane's sudden acceleration nearly left him behind. He could hear Alphonse cry out as Roy scrambled for purchase on the polished metal of the platform, and panic surged through him when he couldn't find a hold. His hips slipped over the edge as his fingers slid along the smooth surface, abruptly catching in a place where two pieces of sheet metal were joined. It wasn't much, but it was enough. Roy clung to the raised seam, heart in his throat, and pulled himself over the edge, crawling shakily away as he gained his knees. Looking back out of the aircraft, Roy was surprised to see that Alphonse was already just a frantically waving stick figure in the distance. Heart pounding and knees still weak from the adrenaline reaction, Roy stood and made his unsteady way deeper into the aircraft.
This might be the biggest mistake of his life, but somehow it didn't feel that way. He wasn't sure if it was just a side effect of the close brush with death he'd just had or something else, but suddenly Roy felt more alive than he had in the last two years. He moved slowly through the flying machine's narrow, debris strewn passage, greatly anticipating the look on Edward's face when he saw his smirking stowaway. Whatever the young man's reaction, it certainly wasn't going to be boring. Mustang felt himself begin to thaw.