A.N. This idea has probably been done before in other Harry Potter crossovers, but if so then I haven't read them yet.
Edward stumbled through the library's dark basement, cursing slightly when the dust he inhaled made him choke. He cursed again when he tripped over yet another box. Behind him, Alphonse caught the back of Ed's jacket just before he fell and easily pulled him back upright.
"I still say we shouldn't be here," Al whispered, somehow managing to stay much quieter than his brother despite his bulky armor. "We can wait until tomorrow and ask a librarian to help us search."
"Screw that," Ed whispered back. "Remember what happened with Central's First Branch? I'm not taking that chance again."
"That was a one-time thing! Just bad timing, really. We're not unlucky enough for another library to burn down on us."
"Yes we are," Ed muttered darkly.
Al searched the wall for a light switch and flicked it on. The library's basement was revealed in all its dusty, stuff-to-the-brim glory. Hundreds of boxes cramped up the tiny space with barely any room between them to maneuver.
"It's somewhere here," Ed said quietly. "The librarian told me that there might or might not be a book on alchemy and the Philosopher's Stone down here. They took it off the shelf because it mentioned human transmutation."
Al looked around slowly, feeling overwhelmed. "Do we have to search through every single box?"
Al stared at his brother incredulously. "We can't get that done in one night!"
"Sure we can." Ed grinned smugly. "Al, you start on that side and-"
The lights flickered and died, leaving the brothers with only the faint moonlight from the windows to see by.
There was a moment of silence and then a resentful, "See Al? We are just that unlucky…"
"Brother, we can come back another day…"
Ed glared at Al's soul-fire eyes, the only part of him that could be seen in the darkness. "If the answer we want is down here, do you really want to waste another night before we find it?"
There was a moment of silence before Al finally gave in.
"But we're not staying here all night, ok? If we don't find anything soon then we're leaving."
Ed smiled brightly. "Then we'll just have to work fast, won't we?"
After some discussion, Ed and Al split up to search different sides of the basement. Once Al's gentle, clanking footsteps had become muffled and faded, Ed turned his attention to his search area and started opening boxes at random in search of alchemy books.
Oddly enough, considering it was a library, there were almost no books in them. Instead they were filled with all kinds of objects, ranging from common household things like kettles and hairbrushes to artifacts so bizarre that Ed couldn't fathom why anyone would keep them, even in storage. At one point he came across a sparkly tiara and a scheme flared briefly in his brilliant mind for a prank involving a certain annoying colonel… But he set the tiara aside with a sigh of regret. He had more important things to do.
Meanwhile, Al trod carefully through his half of the basement, wading through a sea of boxes and past the odd piece of furniture. The furniture unnerved him a little, since they were all covered in white sheets and made him think of ghosts shadowing his footsteps.
He could hear Ed banging into things right and left. Was the basement that dark? Al had noticed that, even though he could tell light from dark, he often had little trouble seeing things no matter what light he was in. It was simply one of the many things his armored body offered as a double-edged sword.
As Al sidestepped a hat stand, his boot caught on one of the sheets covering a tall mirror and pulled it off. He picked it up and walked around to the front of the mirror intending to cover it back up. As he lifted the sheet, he glanced at his reflection absently.
Al yelped and spun around, dropping the sheet in the process. He scanned the darkness frantically, looking for the person he'd seen reflected in the mirror. But the basement behind him was empty.
Slowly, Al turned back towards the mirror and jumped. The boy was still there. Al was about to look behind him again when he realized something was off. Al leaned closer to the mirror's surface.
Ed sprinted towards where he'd heard Al's voice, kicking boxes aside without a second thought for their contents. He skidded to a halt at Al's side, heart thumping painfully in his throat.
"Al what happened, what's wrong?" Al seemed ok. Ed watched in confusion as Al lifted a hand to point at the mirror.
Ed glanced between the mirror and Al. "What? I don't see anything."
"Look!" Al exclaimed, hollow voice filled with excitement. "Brother, look! Look at my reflection!"
Ed looked again, this time taking time to scrutinize the mirror. It seemed normal enough, if a little ancient. It was nearly three times as tall as Ed with a tarnished gold frame. Etched across the top was an inscription in a language he'd never seen before: Erised stra ehru oyt ube cafru oyt on wohsi.
Ed turned back to his brother. "What are you talking about? Al, there's nothing different about your reflection."
Al turned towards him and Ed could have sworn he saw uncertainty and disappointment in those red eyes. Al lowered his hand, his shoulders slumping.
"You…Brother, you can't see it?"
Ed was torn for a moment. One the one hand, he wanted to snap at Al for making him worry like that and calling him for nothing. But Al had sounded so happy for a second, like he'd found something wonderful. And now his voice had that quiet sadness in it that Ed hated hearing. It was like that time when they were children when Al had tried to show Ed his invisible kitten and Al had cried when Ed told him that it was all in his imagination. He'd felt so bad about it that he started pretending to see that invisible kitten just so his little brother wouldn't feel bad.
He couldn't pretend now, but at the very least he shouldn't dismiss whatever Al wanted to show him as fantasy. Ed scanned the mirror once more. "What am I supposed to be seeing?"
Al shifted. "It's…my reflection's different. I look..." Al reached out to him suddenly. "Here, stand where I am. Maybe you'll see it."
Ed complied, stepping before the mirror while Al stood just behind him.
Ed shook his head. "I still don't see-"
Then Ed's eyes bulged. He looked from Al to the mirror, little choking noises coming from his gaping mouth. Finally, he pointed at the mirror and forced out a strangled, "Holy crap!"
The boy in the mirror smiled, silvery-brown eyes lighting up.
"See?" Al cried. "You see it now, right? It's me!"
It really is him, Ed thought as he examined the boy that stood just behind his own reflection where Al's armored body should have been. He had blond hair, slightly darker than Ed's, and same wide eyes and rounded face that Ed remembered. But he was also older, taller, more lanky like a teenager. Ed watched in astonishment as the boy mirrored Al's every move, just like a normal reflection only more uncanny.
Mirror Al looked over at Ed's own reflection and smirked, then moved over to place a hand on his shoulder. At the same time he heard Al giggle and felt a leather gauntlet against his own shoulder. "You shouldn't stay like that, Brother. You'll get dust in your mouth."
Ed blinked and struggled to get something coherent out. "It's really…I can't believe it…you're…"
Ed's eyes narrowed. "You're taller than me."
The reflection raised an eyebrow in surprise. "Am I?" Al asked. "I didn't get a chance to check before, but I guess I would be. I was always at least a little taller."
Ed looked up at Al, feeling a slight sadness when he saw the armor. The mirror had almost convinced him otherwise. "What do you mean? I thought you could see this too?"
Al shook his armored head solemnly. "Once you stepped in front of it, the reflections went back to normal. I think it only works for one person at a time."
"That doesn't make sense...scratch that, this whole thing doesn't make sense! How can a mirror show you as a human? It defies all the laws of science!"
"I don't understand it either." Ed smiled as Al's reflection got that familiar thinking look on his face, the look Al always used to get when he was trying to figure something out. Mirror Al looked up and squinted at the mirror's inscription before his eyes widened in understanding.
"Brother, have you looked closely at that sentence? If you read it backwards it says, 'I show not your face but your heart's desire'."
"Which explains exactly squat," Ed concluded. Suddenly, he grew still. "I wonder…"
Slowly, Ed pulled back the sleeve on his right arm and sucked in a breath. "My arm's back," he breathed. "In the mirror it's flesh and blood again."
"This is so amazing, Brother! Maybe this mirror has some kind of special power! It would have to in order to show someone what they want most!"
"How amazing can it be if all it can do is show you something you don't have?" Ed snapped bitterly. Al looked down at his brother in alarm. Edward lowered his head, hiding his eyes. "What's the use of having everything you want most dangled in front of your eyes with no way to reach it? What's the point of that?"
Al didn't say anything for a moment. He looked back at the mirror, feeling a twinge of guilt. He had thought Ed would be happy to see this, but all he'd managed to do was cause his brother pain.
"I don't see it that way at all, Brother," Al said quietly. "Even if it can't bring us what we want, I'm still really glad that we saw this."
"Al…" Ed hesitated then continued. "Aren't you…angry? Seeing what you should be like now, what you would be like if you hadn't been bound to the armor?"
Stupid Brother, Al thought. "Why should I be mad? Brother…Right now, after seeing myself the way I should be, I feel more hopeful than I have in years. There have been times in the past few years when I would feel…like I was losing sight of our goal, but this kind of puts it all into perspective. It's reminded me that regaining our bodies is worth all that hard work."
When Ed didn't respond, Al asked softly, "Aren't you even a little happy to see me as a human?"
Ed once again looked into the mirror, drinking in their reflections. "Of course I am…" he murmured.
Mirror Al smiled softly and Ed smiled back, his eyes once again tracing those familiar features. He'd forgotten how much like their mother Al looked when he smiled like that.
After a moment, a hopeful look crossed the reflection's features.
"Brother, can I…look again?"
Ed stepped aside without a word and Al stood before the mirror, completely spellbound. Spellbound, Ed thought. That's a good way to describe it…almost like…
And he had to grin at the foolishness of the thought.
"What do you mean it's gone?" Ed snapped at the stunned librarian. "It was here yesterday! Why did you people move it?"
The librarian took a deep breath, obviously struggling to keep her voice calm and level. "Sir…there was never any mirror down here…"
"We saw it!" Ed shouted. He raced over to the spot where he and Al had been the night before, kicking up dust that spun and danced in the sunlight coming from the basement windows. "It was right here! Al, tell her!"
"Miss," Al began hesitantly. "Are you sure you never saw it? It might have gotten moved without your knowing…"
"I don't think so. I'm in charge of keeping everything down here organized and I know I never saw a mirror like the one you're describing. Are you sure it was in this particular library?"
"Of course we are! It was here!" Ed said, striking a box in frustration. "Just about the only useful thing down here anyway, since we couldn't find that alchemy book…"
The librarian watched them for a moment. "Gentlemen, I still have some work to get done. You may look around down here if you want, but please don't move things around without my permission." With that, she walked up the stairs, leaving the brothers alone. Al looked back at his still-fuming brother.
"Al, you can't tell me you're not upset by this! You're the one that wanted that mirror the most!"
Al bowed his head slightly. "Yes but…if it's not here then there's not much we can do about it."
"Maybe we can track it down," Ed said idly. "Someone has to know where it went…"
"Does it really matter that much? Brother, you said it yourself. Sure it's interesting, but it's not of much use to us if it can't give us what it shows."
Ed turned to him. "There was something special about that mirror. I thought about it for awhile and if it can show us what we want, then maybe we can make it show us how to get it! It might even be similar to the Gate. If we just had some time to study it, maybe it would show us what price we have to pay to get what we want..."
He trailed off, looking longingly at the spot where the mirror had been. There was no space between the boxes, no tracks in the dust, nothing to show that the mirror had even been there. Just another lost lead mocking all their efforts. He heard Al's footsteps drawing closer to where he stood.
Ed turned to meet Al's red eyes, wishing they were silvery-brown instead…
"Please, don't beat yourself up over this. Somehow, I really don't think that mirror was the answer we were looking for. We can't let it distract us from our goal."
Ed scuffed the floor. "Maybe…but I still feel bad. I know you wanted to look in it again…"
"I've already seen it once," Al murmured with a hint of a smile in his voice. "I can wait until we get our bodies back to see my reflection. I'm not such a narcissist that I need a magic mirror just so I can stare at myself all day."
He paused and added, "Unlike some people."
Ed's gaze snapped up. "Wha-? Wait, you think I want the mirror back just so I can look at my limbs? I don't miss them that much!"
"Could've fooled me," Al chuckled. "The way you yelled at that poor librarian earlier it was like you lost something way more precious than a mirror…your vanity perhaps?"
Ed opened and closed his mouth several times then scoffed. "You should talk! You spent the most time looking at yourself! I practically fell asleep waiting for you to finish!"
"And who was dragging who back to the dorm so he could get some sleep before dawn? Admit it, you would have stayed here for a week if I'd let you!"
"You didn't want to leave either!"
The librarian was back and she was not amused. "If you're going to raise your voices, may I ask that you take them outside?"
Ed and Al looked at each other and Ed grinned sheepishly. Al giggled.
"Fine, we're leaving," Ed said, then he paused and dashed among the boxes. Seconds later, he returned, stuffing something under his jacket.
"Later," Ed hissed, grinning mischievously.
Three Hours Later
Colonel Mustang shot awake at the sound of a gunshot and the wind of a bullet over his ear. He instinctively rolled out of his chair and scanned around for his enemy, tugging on his ignition-cloth gloves in the process.
It took him a moment to realize he was in his office and that all was deathly silent. He peeked cautiously over his desk and ducked as Lieutenant Hawkeye put a hole in the wall above his head.
"What the hell is your problem?" Mustang bellowed. "For your information, I was not sleeping!"
There was a tiny thump of something light landing on his desk and Mustang risked a glance at it. His mouth dropped open at the sight of a sparkling tiara with a note clipped to it that bore a rough imitation of his own handwriting:
To my lovely lieutenant,
Something to make you stand out and help you discover your inner femininity
Sincerely yours, the Flame Alchemist
Mustang gulped and looked back at his livid lieutenant.
"Well…it does go nicely with your uniform…"
The ominous click of Hawkeye cocking her gun made Mustang, along with everyone in the vicinity, run for cover.