Disclaimer: Harry Potter and all its associated characters are © J. K. Rowling and Warner Bros. I'm not making any money from this little story. A shame really as I could do with some.

Summary: Maurader Era fic. Ever wondered why the enmity between Sirius Black and Severus Snape is so high? No mere school bullying could account for its intensity. Well, imagine that Severus has a twin brother, and imagine that Sirius gets it into his head to seduce him… well this is the story of what happens next. Slashiness with Sirius/OC, hints of Sirius/?? And set in their 6th year at Hogwarts. Enjoy.

Author's note: So this is a first for me in many ways. 1) My first venture into the Harry Potter universe. 2) My first slash story; frankly my first story with any 'romance'. And 3) My first time writing from first person perspective. However, this story seemed to lend itself to that. So it's definitely starting off from Sirius's POV but I'll probably switch perspectives at some point. This story is all planned but certainly not all written out… anyway that's about it. I'm currently half way through the write up of my Master's Thesis and this was a nice little break from that but work is calling again… alas. The title is in Latin for no other reason than I couldn't really think of a snappy title and I like Latin. It means from possibility to actuality.

A Posse Ad Esse

Chapter One: How it all Began.

Sirius

It all started out as a bit of a joke, really; a passing remark made by James one evening whilst the four of us were discussing new ways to wind up Snivellus in this, our sixth year at Hogwarts.

"Y'know what would really wind him up, Padfoot?" said James lazily, lolling on one elbow on my bed and giving me a wink. "If you went about seducing his brother and let the whole school know.'

I almost choked on the pumpkin juice I was swigging. James had often come up with some wild schemes in the past but this topped the lot. Of course my reputation with the laydees was a source of much amusement for my best friend, and he often claimed if I couldn't get the girl it couldn't be done. Obviously he had now broadened his horizons on that front.

Remus merely looked uncomfortable and Peter giggled nervously while I swiftly told James where to stick his idea. He was laughing so naturally I knew he wasn't being serious, but as the conversation moved onto other things I found myself, unwillingly, lingering on the subject.

Severus Snape's twin brother, Tristan, was not someone I had ever really considered before, except in passing. Of course as a Slytherin in our year he attracted the usual Marauder pranks and general scorn but most of our attention was naturally focused on our arch-nemesis- Snivellus.

Lucky for Tristan I suppose.

That night I lay awake for sometime, thinking over what I knew of him. Quiet and studious, Tristan Snape did not seem to be someone who rose to bait easily, generally ignoring our humorous jest and japes; he didn't play Quidditch so there was no rivalry there, and I had certainly never seen him as competition in the girl stakes. Mind you, there really wasn't anyone I considered competition there. I was almost 17- young and cocky, and pretty confident that I could get anyone I wanted: girl or boy. It made little difference to me at this point as I was hardly ever thinking further than a bit of necking and fondling in a broom cupboard… of course there had been one or two exceptions… Anyway nothing would even go that far in James's prank- I just had to get the guy to like me, and Merlin knew, I was good at that.

It was obvious the next morning that my friends had forgotten all about last night's joking, but as they chatted over breakfast I found my eyes scanning the Slytherin table searching for my quarry.

As luck would have it, he was just getting up to leave as I found him, delayed in chatting to some friend who was fumbling with his satchel. This afforded me a clear view across the rows of heads that separated the Gryffindor and Slytherin tables, and I took full advantage.

It was obvious at first glance that the Snape twins were not identical, they certainly didn't look that much like each other. This was a good thing as I don't think I could have coped with trying to seduce a carbon-copy of Snivellus- it would have been too wrong for words.

It seemed that Tristan had come out on top in the looks department too; also lucky because there was no way Snivellus was going to win any beauty awards in my opinion, not with his obnoxiously large nose and greasy hair. His brother, on the other hand, was not bad looking in a quiet kind of way- not a stand out beauty of course, but I had, in the past, had a thing for the quiet ones… Dark haired and dark eyed, in that he resembled Snivellus, but although his skin was pale it lacked the sallowness. And he had good teeth. This I had noticed before because I had a bit of a thing about teeth: there was no way I was sticking my tongue into an ugly mouth and that was that. No problem here though because Tristan's teeth were white and straight and perfect- excellent.

I'm not quite sure why I made up my mind to do this thing. My innate sense of mischief, perhaps, because frankly I could think of nothing that would drive Snivellus up the wall more. (Well, except James getting with Lily and let's face it, that ain't gonna happen.) Or maybe I was just curious, eager to test my own prowess. Could it be done? Only one way to find out…

Well whatever the reason, I pondered about it all day and by the time it was just me and Prongs, Moony and Wormtail chilling by the fire in the common room, I was fully resolved to try it out.

"I'm gonna do it," I announced suddenly during a game of chess with James.

"Do what, mate?" he asked idly, scrutinising his pieces intently. He had no chance anyway, I was two moves off checkmate and he hadn't a clue. Chess was never really James's forte.

"Seduce Snape's brother of course, whaddya think?" I answered triumphantly, confident in the shocked responses I would get, and secretly amused. Peter squeaked in what was probably horrified excitement and James's head whipped up, jaw dropping. Behind me I heard Remus close his book with a thud. He said nothing though.

"Are you serious?" James asked. He looked gobsmacked, it was all too amusing. I laughed.

"Yeah sure, why not? Don't look so shocked mate, it was your idea." I grinned at his expression.

"Well yeah, Padfoot, but it was a joke. I wasn't actually serious!"

"Oh I know, but I am. Just think of Snivellus's face when he finds out. It will be incredible!"

"Playing around with people's feelings is never a joke, Sirius." Said someone quietly. It was Remus of course.

"Ah lighten up, Moony, this is Snape we're talking about." I said, swivelling to face him. "He hardly classifies as people."

"And his brother?" Remus challenged coldly, his hands quite still on the front cover of his book. "Does he classify as a person, or is he just another unwitting pawn in your chess game of life?"

I smiled easily, not in the least bit bothered by Remus's attitude. He was always trying to act as my moral compass. "He's a Snape, same difference." I replied. My friend's expression altered not one jot so I decided to lighten the mood. "Pssh chill, Remus. I know you're just jealous really; wish I was seducing you instead, eh?" I gave him my best salacious wink and he rolled his eyes, reopening his book. "Oh, shut up, Sirius."

Conversation closed, it seemed. And argument won. Remus had never been able to argue with me.

James shook his head as I turned back to him. "You're a nutter, mate, but I'm not gonna argue with you."

"Good!" I said, leaning forward. "Wanna make things even more interesting?"

"A bet?" he asked, suspiciously.

"Ah ha, good to see we're on the same wavelength, Prongs me old chum. A bet indeed. Five galleons? If I can seduce him before, oh…, say, Easter?"

"Easter?" James gave me an incredulous look, "That's like six months! I hope you can act a bit quicker than that, Padfoot!"

"Oh fine!" I huffed. This would be tricky work, couldn't he see that? "How 'bout Valentine's Day? Nice and symbolic, don'tcha think?"

James rolled his eyes, echoing Remus's earlier gesture. "Alright, fine. Five galleons. But I have to tell ya, Sirius. Rather you than me on this one!"

But sitting there in the common room, in the warmth of the fire and surrounded by my friends, I felt brimful with confidence. The mission was on and I wasn't going to back down.

So there's chapter one, hope you liked. I shall hopefully be back soon with an update, depending on how much stupid thesis gets done. See ya!