Just something that popped into my head. Inspired from Stephenie Meyer's 'New Moon'.
Alphonse Elric had a theory as to how to summon the hallucinations.
Recklessness plus stupidity equaled Edward.
His brother's voice would be yelling within the deep depths of his mind, very soon.
It was about a year after he regained his body back that he began having the hallucinations. He was staying with Pinako and Winry, always moping around and trying desperately to cope with life...without his older brother.
Life without Ed was horrible. There was no one to talk to, no one to fight with, no one to comfort him. There were no arguments between Ed and Winry, and milk in the fridge became more prominent.
It was if Edward's very essence was embedded into Alphonse's life. And there was no way to rub it off.
The only solution had been to keep shining his essence like tuning up a piece of metal.
And that was how Alphonse began doing dangerous things.
Edward had made him promise, before he left, to not do anything stupid.
Al, don't bring me back if anything happens to me. Don't do anything dangerous. Don't try to save me. I'm just fighting the last of the Homunculi; don't wait for me, understand?
Al promised, being the good little brother he was, but when Edward did not return he made a vow to break that promise.
Why?
Ed promised he'd return.
And he didn't.
This made Alphonse angry; this made him uncharacteristically enraged.
The first time he triggered Edward, it was in the middle of a clearing in the outskirts of Resembool, in a thunderstorm.
Alphonse knew the lightning would hit him if he stood very still. He was the tallest thing in the clearing; it would be bound to hit him.
Standing there, arms outstretched and palms facing the heavens, he let himself be drenched with rain as he waited eagerly for the lightning to strike. To stab his heart.
Or at least what was left of it.
There was a piercing crack in the sky, about a few feet away from Al. Al swore, and taunted the storm into hitting him.
"Give it your best shot!" he grinned as if nothing was wrong.
You broke your promise, Brother. Now it's my turn.
"Al?!"
Al started, flinching at the sound. It was as clear as day, like it was next to him.
But when Alphonse looked beside him, all around him, there was nothing.
"Al, what the hell are you doing?!"
It was Edward. Oh, God, it was Ed.
Tears of anger, sadness, grief, agony, and joy filled Al's eyes as he answered the voice in a low whisper,
"Brother?"
He feared speaking normally, for if he did, he felt it would take the magic from everything.
"Alphonse, go back to Winry! You're being stupid!"
Al smirked, "And if I don't?" his voice was still a whisper.
"If you don't, I'll kick your ass!"
"You've never beaten me." Al shook his head.
"Idiot, go back!"
"No."
"Alphonse, don't you dare!"
It was then that Alphonse noticed the storm clouds overhead were gone. The sky was clear once more.
"Brother?" Al questioned.
There was nothing.
But this didn't discourage Al. Since that day he continued doing reckless things; each time it triggered his brother's voice.
Maybe Ed didn't have to be gone completely...
Alphonse Elric had a theory as to how to summon the hallucinations.
Recklessness plus stupidity equaled Edward.
His brother's voice would be yelling within the deep depths of his mind, very soon.
Like an alcoholic to beer, Alphonse grew addicted. Each and every day he tried to do stupid things just to hear his brother again. Whether it was something as big as trying to strangle himself or as tiny as deliberately getting a paper cut, it didn't matter, Al would do it.
Winry and Pinako began to watch him more carefully. Al began thinking of careful tactics, strategies, ways to avoid them.
People were starting to question his sanity. But Al didn't think he was crazy. He couldn't be. Was it wrong to simply have a piece of someone you love with you...always?
"Al."
Alphonse shut his eyes in satisfaction and let out a breath he didn't even know he was holding, "Yes, Brother?" he inquired innocently.
"I'm warning you, Al," Edward growled deep in his head, "don't you dare do this."
"You broke your promise," Al mumbled, "now watch me break mine."
The barrel of the gun he'd found in Pinako's drawer clicked beautifully, and Al pressed the gun more forcefully against his forehead.
"NO!" Ed yelled, "AL, DON'T!"
"Why not?!" Al yelled at nothing, while Winry and Pinako knocked furiously, anxiously, against the locked basement door, "You told me, you promised you'd come back alive! You didn't! You're dead!!"
"AL!"
"Brothers keep their promises," Al spat angrily.
"Al, I love you, don't do this," Ed's voice was a whisper now.
I love you?
Alphonse's shoulders began shaking uncontrollably, and then he roared with laughter,
"Don't give me that, Brother. If you loved me, you wouldn't have broken your promise!"
"Alphonse!"
"Edward!" Al teased, clicking the gun once again.
"AL!"
"ED!" Al yelled back, sounding like a little boy. Similar to when they fought as children.
"NO!" Ed screamed.
"YES!"
"ALPHONSE!" Ed sounded like he was crying now.
"Brother," Al chuckled, "you idiot."
And then he pulled the trigger.
Edward had simply been reported missing after the explosion in the underground city. He was rumored to be dead.
He returned to Resembool, alive and well, about a year after Al killed himself.