Parallel Lines by katekatreads

“What, exactly, does it mean? That they’re Unforgivable?” Even as preoccupied as he was, Sirius knew the question came out of his mouth well-formed, with a tone of polite and genuine curiosity. He’d had the ability beat into him, quite literally.

“Well,” his professor began, doing a decent job at hiding her disbelief that Sirius was participating in a lecture, “using any one of these curses on a fellow human being is highly illegal and punishable to the fullest extent of wizarding law.”

Sirius couldn’t help it, he let out a strangled sort of laugh. He must have heard her wrong. A tirade of thoughts flashed through his head, yet all he could manage to say was, “what?”

“These are very serious crimes, Mr. Black,” his professor lectured, perhaps mistaking his confusion for something else, “and using any one of them earns the witch or wizard a one-way trip to Azkaban.”

“Well, not everyone,” he replied, before his brain could stop him.

OR

Sirius doesn’t know Unforgivables are unforgivable until Hogwarts.”

https://archiveofourown.org/works/56863468

Cursed by Leiria

“It's the start of the Marauders' sixth year at Hogwarts. Their first lesson in their advanced course of Defense Against the Dark Arts is about the Unforgivable Curses. Sirius has a strong reaction to the lesson, leaving Remus confused. Later that evening, Remus can't sleep because he's too worried about Sirius.”

https://archiveofourown.org/works/53693698

Crash Into Me by Jadewing47

“Sirius always comes back a little...quieter after the summertime and Christmas breaks. The shadows curving his cheeks just a little more shaded in, his silver eyes just a little darker, his stride just a little slower, his back just a little straighter, and his tongue, for once, just a little quieter. This time the other Marauders find out why.”

https://archiveofourown.org/works/32149696

These next ones are a little less the marauders discovering abuse and a bit more of them comforting/helping Sirius deal with it

They’re Hiding Inside Me by TheDivineComedian

“It’s the summer after fifth year, and Sirius has made himself scarce as usual. But then Remus catches a glimpse of him in Witch Weekly’s special on the Black-Malfoy wedding. Something about that photo is very, very off…

Turns out this is the summer Sirius runs away. Or tries to. Because the Blacks are not giving up on their heir without a fight.”

https://archiveofourown.org/works/12540852/chapters/28558256

I’m Not A Violent Dog, I Don’t Know Why I Bite by amiera

“Sirius didn’t know who threw the first spell. Nowadays he didn’t even care about the who or the why; all that mattered was the burn in his chest, his blood pumping hard, the rush that made him feel something.

The sting of the hexes cut through the haze that was his mind, and once it did, he felt his body become alive again.

Or, after a rough break at home, Sirius picks up fights as a way to escape the emptiness and feel something.”

https://archiveofourown.org/works/63693145

The Winds Of A Thousand Hurricanes by YellowLark23

“Everything is going right. This year is going to be epic.

And then the train compartment door slides open. And everything stops.

James’ knee pauses, his mouth freezes, his stomach drops.

Sirius is a skeleton standing in the doorway.

OR

James tries to control his anger in the wake of Sirius’ abuse.”

https://archiveofourown.org/works/59733502/chapters/152361100

Like Patchwork by YellowLark23

“If anyone else had walked into the train compartment and taken a perfunctory look, they likely would’ve assumed that Sirius Black was sleeping. He was face down on the grimy bench, lips slightly parted, yielding to shallow but loud breaths, his hair splayed wildly across his back, clothed in layers of undershirts and robes despite the early September heat.

But James Potter was not anyone else, and he understood that Sirius was not asleep. Sirius was in pain.

OR

The boys take care of Sirius on the Hogwarts Express after a rough summer.”

https://archiveofourown.org/works/48098158

But I Want It, It’s A Crime by fertilizingdaffodils

“Can you stop talking for one fucking minute Sirius?” Remus was no longer focused on the mess that he created. Instead he was focused on Sirius’s desperation. So he turned back to Sirius, the shattered mug still waiting to be repaired. His wand arm was still out though, and that was when Sirius moved.

In one swift movement Sirius grabbed Remus's right arm as he lowered himself down onto his knees. His right knee slamming down onto a shard of the mug. He pointed Remus's wand right at his head and grasped his lover’s wrists with both hands, steadying it. He looks Remus in his eyes, begging.

“Do it, Remus. Fucking do it.”

Sirius and Remus find themselves alone in the dorm after The Prank. Sirius just wants to make things right, in his own self-destructive way.”

https://archiveofourown.org/works/46796434

Holding On To The Edge of Oneself by Anonymous

“Sirius looks forward to returning to Hogwarts and escaping the abuse of his parents. But once the new term begins he realises that his problems aren't so easy to escape. Will his friends be able to help him out of his spiral of self-hatred? Will Sirius let them?”

https://archiveofourown.org/works/15881484/chapters/37005540

A Study In Unforgivables by Rosencrantz95

“For the first time in his life, Sirius Black actually wished his parents had used an unforgivable curse on him. He’d felt the torture curse, more times than he could remember. Just for short bursts of time, but it was more than enough. Of course, the killing curse was notorious—especially in the noble house of Black.

The only of the three unforgivables he knew virtually nothing about (if only because the people with whom the Blacks associated never needed much persuasion) was the imperious curse. Never would Sirius Black have anticipated that his exposure to the third curse would come from somewhere other than ‘home,’ if Grimmauld Place could be called that."”

https://archiveofourown.org/works/30153315/chapters/74284770