Once upon a time, there were two separate worlds. And like all separate worlds these coexisted harmoniously until a single event split them permanently onto parallel tracks that would never again meet, allowing each world to grow independently of the other, becoming more and more vastly dissimilar as time went on. The crux of these two worlds, that moment when they first split, occurred in the rather extraordinary great hall of a rather extraordinary school in Scotland, where a man gazed upon the form of a small boy.

This man was named Severus Snape. He was a bitter man. A spy who had seen and done much wrong. He was also a man who had lost the woman he loved to the man who had tormented him in his already-miserable childhood. And the boy whom he watched was the product of that union.

And up to the break in the continuum, Severus Snape’s thoughts were these…

Severus Snape looked down a long, hooked nose at the cowering first years. His black eyes settled on a particularly scrawny specimen and, in a conditioned response that would have made Pavlov proud, felt a surge of animosity at the sight of a perfect replica of his childhood nemesis. Slight build, check. Disgracefully messy black hair, check. Ridiculous knobby knees, check. Black, round-rimmed wire glasses, check. Arrogant, hazel eyes…

The boy he had been scrutinizing suddenly looked up, his eyes locking on those of the surly professor, and Severus Snape barely suppressed a gasp.

Lily.

Only years of spywork allowed Snape to retain his composure as he casually broke eye contact. Damn him. He had her eyes. Her beautiful, shimmering, emerald green eyes shone out of the wretched, arrogant face of James Potter. Snape’s mind screamed its indignation. Did he really have to spend the next seven years gazing upon this reminder of his mistakes, the proof that the woman he loved had married…had loved…the man whom he had hated?

The child his Lily had give her life to save?

Rage broiled dangerously beneath his cool façade. She had been his Lily, damn it. She was his best friend long before she even met Potter. Merlin, she had hated Potter before sixth year. What right did James Potter have to sweet, caring, radiant Lily? With his arrogance, his cruelty, his self-serving ways, what right did he have to her as his wife? What gave him the right to have her in his bed? To have her be the mother of his child?

His child.

He searched again for that messy mop of hair, but did not see the boy. His mind only showed him the ghosts of long-buried memories. He had loved her. Always loved her. Would have done anything for her. She should have been his. She should have been his wife. She should have shared his bed. The boy should have been his child.

His child.

Here is where the world splits. In one universe, the knowledge of that lost opportunity fills Severus Snape with a powerful bitterness. He takes the opportunity, the next time the child looks at him, to scowl darkly. He spends the next several years punishing the boy for the sins of his father, while simultaneously protecting him for the memory of his mother. Eventually, Snape meets his end, staring into eyes that remind him of his long lost love, alone in all the ways that count and with a heart filled with regret. Blood, venom, and memories, the things that wholly constituted his life, pooled around his dying body.

But that story has already been told, and is in fact not mine to tell.

My story is that of the second universe that arose from that split, and in that second universe, events unfurled quite differently.

…The boy should have been his child.

His child.

And he’d be damned if he was going to have something of his taken from him again. Snape smirked. It was high time that James Potter, the spoiled sod that he was, even dead, had something precious taken from him.

He would roll in his grave to see his heir look to Snivellus for guidance. James Potter’s son would be the son that should have been his…His child with his beloved Lily Evans. The ultimate revenge. He would steal the boy’s love away from that bastard who’d stolen hers from him.

His decision made, he immediately put all of his Slytherin cunning to a plan. And when the boy met his gaze again, Severus Snape did something he had not done in many, many years. He smiled warmly at the boy he was determined to make his own.

Fleetingly, and with a hint of confusion, Harry James Potter smiled back.

Harry Potter stared, shocked, at the menacing man who had just smiled at him. The man looked how he always imagined a vampire would look. Dark, brooding and tall, too pale and too thin, and with eyes that seemed too black to belong to a living being. And yet, when he smiled, Harry couldn’t help but respond. He had been shown too little kindness in life to look such a gift as a smile in the mouth to see if it had fangs.

“Who’s that?” Harry whispered to Ron. But Ron wasn’t paying attention, too focused on watching the bossy girl from the train and hoping she didn’t get into Gryffindor.

“His name’s Professor Snape,” a voice from behind Harry said in a surprisingly mellow, low voice for an eleven year old. Harry turned to see a slim boy with dark skin looking at him appraisingly. “He’s the potions master and the head of Slytherin house. He was accused of being a supporter of Voldemort in the last war, but was acquitted. Supposedly Dumbledore vouched for him…told the entire Wizengamot that he was a hero and a great man.”

“But don’t you have to be a Slytherin to be head of Slytherin house? I thought everybody who went into Slytherin was evil?” Harry exclaimed.

“Evil?” The dark by grinned. “Who told you that? Weasley there?”

At the sound of his name, Ron whirled around to face the boy. “What are you saying about me?” he snapped. Harry glanced at the furious redhead nervously while nodding his head. “Yeah…but not just him. Hagrid told me there never was a wizard who went bad who wasn’t in Slytherin. I mean, Vol…You Know Who…was a Slytherin, right?”

“He said there wasn’t a wizard who went bad who wasn’t in Slytherin, not that all Slytherins went bad. Besides, I doubt that’s true. I don’t know this Hagrid bloke, but Slytherins have been prejudiced against for years.” The boy sneered. “Awful hypocritical, don’t you think Weasley, to hate someone for something they can’t control? Like the house they were sorted into? Or for their lack of money? I heard that prat Malfoy on the train.” Ron flushed a bright red.

The boy turned back to Harry. “Look, Potter. I’ll tell you something my mum told me. Slytherin is not a house for those who are evil. It is a house for those who are very adept at wearing masks. It’s not their fault that the world is naturally suspicious of masked men.”

Harry’s mind flashed through years of torment, of pretending gratitude towards a family who locked him in a cupboard. Of playing dumb so he didn’t get better grades than Dudley. Of hiding his anger and hatred of the Dursleys so he didn’t get the shit beat out of him. He knew about masks.

Ron nudged him, hard, and broke him out of his reverie. “They’re calling you, mate.” Harry started and then nervously took his place on the stool amidst a sea of whispers so thick as to have its own air current. The world went black as the too-large hat was lowered over his eyes.

“Hmm. Difficult. Very difficult. Plenty of courage, I see. Not a bad mind either. There's talent, oh my goodness, yes - and a nice thirst to prove yourself, now that's interesting...So where shall I put you?” The hat paused, as if waiting for Harry to comment. “Distracted, are we?” it asked. “Your thoughts are in quite the turmoil over what that boy said. He was right you know, Slytherin is a house for cunning and ambition, not a house for evil. Does no one listen to my song? Evil is hardly present in eleven year old minds, and I can assure you that there has been evil in all four houses. If there have been more in Slytherin…well…we often find ourselves fulfilling the parts others expect of us, don’t we?”

“I don’t understand,” Harry thought quietly to the hat.

The hat chuckled. “Well, we can hardly expect an eleven year old to process such abstract philosophical comments.”

“Sir? May I ask a question?”

A full out laugh this time. “Sir! I’ve never been called that, I’m sure. And you were wondering about Professor Snape?” Harry nodded. “Ooh, don’t do that boy, makes me seasick. As for Severus, I’m not sure why he smiled at you…but you two boys have much in common…much in common indeed. He could help you, Harry. Just as Slytherin could help you on the way to greatness, there’s no doubt about that. What do you think, Harry? Is Slytherin the place for you?”

And before Harry could answer, the hat shouted for the whole hall to hear…

“SLYTHERIN!”

Shakily, Harry lifted the hat off his head and placed it back on the stool. Unlike previous children, his sorting was met not with loud cheers but stunned silence. As he walked to the correct table, certain faces stuck in his mind. The, (ironically), identical expressions of shock and horror on Malfoy’s and Ron’s faces. The cool glance of the dark-skinned mellow-voiced boy. The stunned but oddly smug expression of Professor Snape. The oddly calculating stare of Professor Dumbledore. Harry sank into a seat as far from Malfoy and his thugs as he could get, not paying attention to the rest of the sorting until the boy from before sat down next to him and extended his hand.

“Welcome to Slytherin, Harry Potter. I’m Blaise Zabini.”

Harry looked at the boy and smiled nervously before taking his hand. “Thanks…and it’s just Harry.”

It is an interesting fact that the same method of walking used by muggle supermodels to make their hair fan out on the catwalk was used by Severus Snape to make his robes billow behind him. No one would ever venture to voice this concept to the man if they valued their lives, but nevertheless it remains valid. He did it for much the same reason as those twiggy women do, as well: effect. Snape was a huge believer in effect.

He knew that his billowing robes made him look dangerous and impressive, adding to his perceived size much as certain snakes release their hoods to scare off enemies. He also knew that speaking quietly made people listen more carefully for fear of missing something. He knew that black clothing allowed him to blend in the shadows and force his students to always wonder if he was nearby, watching. He even knew that a proud demeanor towards everyone but the dark lord made him seem as if he was only subservient to him, as opposed to generally weak, and got him less time under the cruciatus.

But the effects of a simple smile were well beyond even his grasp. As the feast ended Snape's mind whirled as he stood from his place, quickly disassembling and reassembling his plans to account for this newest development. A Potter in Slytherin - whoever would have guessed? Obviously, the centuries-old hat was going senile...or perhaps it had spent too much time around the perpetually nutty headmaster? No matter. This just added another layer to his already sweet revenge. At this rate, James would not just be rolling in his grave, but thrashing.

Coming to the stretch of dungeon wall that hid the Slytherin common room, Snape snapped out the password, ex malo bonum, and entered.

"I am sure Deputy Headmistress McGonagall has already explained to you all that your house is ‘like your family' here at Hogwarts," he said with a slight sneer. Silence immediately spread through the bustling common room as the new house members looked at him in fear and the older house members looked at him with the same as well as a healthy amount of respect. He watched the Potter child particularly carefully out of the corner of his dark eyes. The boy's eyes were wide. His sneer increased slightly. "As nauseatingly maudlin as the sentiment is, it is nonetheless true. And as some of those of you from more respectable families will know, loyalty to ones family is paramount." Snape gave a nod to the heirs of the families with purest blood, knowing better than to lose his cover even years after the last sighting of the Dark Lord.

"As such, I expect to see you Slytherins put up a united front when out in the school proper. While in here you are, of course, allowed your disagreements...though I expect you all to refrain from doing anything as common as brawling when in your common room and dormitories. Outside this dungeon, however, you are to show no signs of discord. And above all, if one of you is attacked by a member of the opposite house, I expect that all members of this house to rally against the attacker. There is strength in numbers." He saw a dishevelled black head nodding fervently in his peripheral vision. "Gryffindors will likely harp on this being an unfair fight. Being in Slytherin is being on the losing end of an unfair fight. The school is against you: right up to the headmaster, for all his talk of school unity. Your punishments will likely be harsher than other students from all teachers, with the exception of Professor McGonagall who, while she is more lenient on her students (especially when it comes to Quidditch) will at least not punish you harshly.

"I will, to make up for this, favour Slytherin heavily when others can see. Do NOT think you will escape detention with me, though, at a later date. Other houses will not be aware of your punishment, and house points will not be affected, but you will know the consequences of misbehaving, believe you me."

Snape sent his most baleful glare around the new students and the students who had given him the most trouble in the past, making sure this sank in. The Potter boy was vaguely pale. That's right, boy. You will be disciplined. You will understand cause and effect when I'm done with you. "You are all expected to keep up with your studies. Those who struggle with classes will be assigned a tutor from an upper year. I also expect a certain level of personal comportment and cleanliness. You are to act with respect in regards to your professors and you are to present yourself well in dress and hygiene. Remember, how you look and act is a reflection on Slytherin house and on me." The brat was shuffling his feet now...probably showered once a month, the cretin.

"Maps of the school are impossible to create, given the Unplottable charms on the school*. However, a seventh year will escort you to each of your classes the first week, after which you are expected to find them yourselves. Tardiness due to ‘being lost' will not be tolerated. If you have any troubles, you are expected to first speak to your prefects..." he nodded to the six students in question who rose their hands obligingly. "...and if the situation is direly serious you may talk to me at anytime. My office is down the hall, behind the painting of two snakes intertwined. A brief knock to the door will alert me during office hours and will also trigger an alarm in my private rooms at other times. If you interrupt me at these times without good reason I will be displeased. However, while you are here, I act in loco parentis...as such, I am responsible for your welfare. I do not want you to hesitate to find me if you do have good reason.

"Now, it's been a long night, and you all have class in the morning which you will NOT be late to. So everyone is to go straight to bed right now. First years, the fifth year prefects will now show you to your dorms. Good night."

And Severus Snape left.

As a fan of effects, Snape was also interested in the causes that create them. So it's hard to believe that he did not delve into the possible causes of all the strange reactions Potter had during his speech, or his actions in the dormitory that night. Or maybe not.

Cause, his entire focus was on his revenge on James.

Effect, he saw not Harry Potter, but James Potter's son, object of his revenge.

And so, Harry Potter went off to his new dorm.

Not twenty minutes later, he had a detention. Because he'd punched Draco Malfoy. Because he thought Malfoy would hit him. Because Malfoy approached him in a threatening manner. Because Potter had insulted his family. Because Potter was embarrassed. Because Malfoy'd made fun of his ratty bedclothes. Because they were all Potter had. Because the Dursleys did not provide their nephew with proper clothes.

Cause. Effect.

Too bad Snape could, for now, only see the effects.