Crossposted from my ao3 account.
Full Summary: Sasuke watches Itachi kill their parents in cold blood and he cries and rages, but in the back of his mind he's confused. He sees the coldness in Itachi's eyes and the contempt on his face, but his soul aches. Itachi's emotions are a whirlwind of fear, regret, and they feel to Sasuke as if they're losing themselves in guilt. The emotion Itachi is giving to Sasuke does not match what he's trying to emote from his face.
Itachi leaves Sasuke in a pool of blood that doesn't belong to him and a clan of only one.
Warnings: Sibling Incest
Chapter 1: to flee
Sasuke likes birds. They sing in the morning and he's not even angry that they wake him up because their voices are just so pretty. They tweet in a jovial manner and they know the true meaning of freedom. They soar in the sky and go places that he only wishes he could see.
Sasuke likes song birds and twittering, tiny birds, but his favorite birds are crows. He's heard other clan members complain about what a nuisance the birds can be, but Sasuke loves them. Their caws aren't as pretty as the other birds, not as soothing, but crows are loud and strong and whenever Sasuke hears them, they remind him of Itachi.
Crows are important to Itachi, so they're important to Sasuke. Crows define their bond, so Sasuke will have no other bird as his favorite. After all, it is a large flock of crows that are spread out across his back.
He's only seen them once. The skin on his back had always been darker than his natural skin tone, always hinting that his soulmark would be large and that it would show up before too long. He's playing in the bath, his mother gently washing his back, when he suddenly feels a burning. He cries and whines at the sensation, he's all but two and he doesn't deal well with pain (not yet, anyway) and his mother's distressed gasp just makes him more uncomfortable.
"Kaa-san!" Sasuke cries, slapping at the water. "It hurts!"
"Shh," his mother hisses, her eyes not leaving his back. She picks him up out of the water and quickly towels him off, deliberately ignoring his squirming.
The ache in his back has already died out and Sasuke would've forgotten all about it except his mother lightly ran a hand down his back and started to cry.
"Oh no! Kaa-san?" Sasuke asked inquisitively, concerned at seeing his mother's sad eyes.
"My poor baby," is what his mother says over and over again. She rearranges him so he has a body length mirror behind him and a simple hand mirror in from of him.
This is the first and last time he sees his soulmark.
The crows are simultaneously huge on his back, but so incredibly small due to his size. They start on the right side of his lower back and they look as if they're taking flight. They stretch their wings and flow all the way to the other wide of his back, before swinging and taking a sharp right to have the biggest crow's beak end right where his back and right shoulder meet.
Sasuke thinks it's beautiful. He likes birds already, but his brother's favorite birds are crows, so Sasuke is even more excited at the prospect. Then Sasuke tilts his head a little, his young brain trying to figure out where he's seen this pattern before.
His brother.
Sasuke's face breaks out in a smile and he knows his eyes are sparkling with delight. He shares his soulmark with his brother. That means he and Itachi will always be together. He finally manages to tear his eyes way from his flock to look at his mother and his mood abruptly turns south.
She's crying more than she was before and it registers to Sasuke that they aren't happy tears. His mother reaches for the first aid kit that lies beneath the sink and then she proceeds to wraps gauze around Sasuke's chest and back. It wraps around him and soon he can no longer glimpse his soulmark in the mirror.
"Kaa-san?" Sasuke asks, confused and upset.
"No one can ever know," she whispers, taking his face in her hands. She gives him a sad smile before taking him to be dressed in his room.
For the next few years he is ignorant to why his mother reacted the way she did. Sasuke wants to tell Itachi about his discovery, but he remembers his mother's sad eyes and chooses to refrain. It's when he's six years old and hears his father talk of arranged marriages that he understands.
His mother holds him as he cries that night. His Itachi was to be married to whomever their father chose. Soulmarks would play no role in the decision.
"Why can't he be with me?" Sasuke asks as he sobs into his mother's hold.
"You're brothers," she replies weakly, like the argument would hold any weight.
Sasuke doesn't care if they're brothers. They're meant to be; the matching marks prove it. He cries and despairs for the next year, knowing that he can't share why he's so miserable.
His sadness is what makes it take so long from him to realize that somethings wrong with Itachi. Sasuke has always been able to feel something of what Itachi feels. Some soulmates work like that while others don't. Sasuke knows that Itachi doesn't experience the brief flashes into Sasuke's emotional state like Sasuke can do to Itachi. When Sasuke finally realizes that the crippling dread and fear isn't coming from him, it's too late.
Sasuke watches Itachi kill their parents in cold blood and he cries and rages, but in the back of his mind he's confused. He sees the coldness in Itachi's eyes and the contempt on his face, but his soul aches. Itachi's emotions are a whirlwind of fear, regret, and they feel to Sasuke as if they're losing themselves in guilt. The emotion Itachi is giving to Sasuke does not match what he's trying to emote from his face.
Itachi leaves Sasuke in a pool of blood that doesn't belong to him and a clan of only one.
Sasuke doesn't talk for a long time after that. He doesn't understand what's happened. He gets that his clan is dead, that Itachi did it. He knows that happened because he saw it with his own eyes. But his heart, his soul, knows something else is going on. Something was wrong that night. Itachi didn't want to kill anybody, Sasuke knows that down to his bones, so it perplexes Sasuke as to why Itachi did it.
Sasuke keeps his soulmark covered at all times. He alternates covering it up with gauze and bandages. He's gotten so good at it that he can close his eyes and only occasionally get glimpses of wings taking flight.
He almost let's himself drown in his sorrow, in his confusion and betrayal. He feels the empty space where Itachi should be, feels the coldness that is a constant pulsing in his chest. He should give up on ever understanding why Itachi did what he did. He should focus on being the best shinobi he can be, focus on being better than the traitor his soulmate turned out to be.
Then he meets Naruto Uzumaki and his convictions change.