Disclaimer: I do not claim ownership of Harry Potter, its characters, locations, or names. All of them belong to JK Rowling.

AN: This is my story, For Enemies. It does have a major HPB spoiler that is important to this (namely, who the Half-Blood prince is.) so for those that don't want to know, haven't read the books, or aren't far enough in to know yet…do not read.

- For Enemies -

The explosion shook the tabletops, as the bubbling potion splashed over the sides of the cauldron and on its brewer. Laughter echoed through the dungeon classroom, as the small black-haired boy glowered, turning his gaze over to the table of four boys that was nearby. Two of them had just done a high-five, while one of them looked exasperated and the other fearful.

Seething, he turned back to the cauldron, wanting nothing more then to get back at the group, who were known as the Marauders. "Mr. Snape, go wash up and change your robes," the professor said, glancing in his direction.

Humiliated, fifth year Severus Snape shoved his sodden books into his bag, and trotted out of the room, the laughter of his classmates following him until the yelled "Quiet!" silenced them. Grinding his teeth, he stomped up the stairs, and took a corner to head in the direction of the Slytherin common room.

He knew that James Potter and Sirius Black had been responsible for it, most likely having thrown a firework of some sort into his cauldron. It was common place for them to go out of their way to make his life a living hell.

The few students lounging in the common room didn't look up when he passed, dripping the potion on the stone floor. Severus relaxed seeing the dormitory was blissfully empty, as he peeled his robes off and tossed them on the floor.

As he washed up, as idea came to his mind. He had created his own spells, one of which had spread through the school – and was in turn used against him, ironically – and he knew he could do it again. Severus looked in the mirror, a smirk on his face. James and Sirius wouldn't know what hit them.

- - -

Over the next fortnight, every evening after classes Severus sat in the library, a stack of books on his table, each one of which he was taking turns looking through. Most of them were for charms and spells, while others were for hexes and jinxes. His quill scratched on the parchment as he copied some down, along with the small details that could later be of some aid.

When he finally had enough references, Severus took to staying in the Room of Requirement. He could test the spells effects on the pillows conjured in the room, and make improvements and changes as needed. By the end of the first night, most of the original parchment was riddled with crossed out incantations.

It took several days of going to class during the day and staying in the Room of Requirement at night before Severus finished. When there was only one spell left, he faced the pillow that was propped up, and lifted his wand. "Sectumsempra!" he cried.

Feathers flew about, as it was cleanly cut apart by an invisible blade. Shivering in excitement, Severus pulled out his potions book – Advanced Potion-Making – and wrote the spell down, putting "For enemies" under it in neat handwriting.

The marauder's would not have the last laugh.

- - -

Potions came along fast enough the next day. Voices echoed through the hall, as the students waited for the teacher. Severus leaned back against the wall, watching the notorious group of four. James met his eye, and they glared at each other. "What are you staring at, Potter?" he asked.

Sirius eyed them, before looking darkly at the Slytherin. "At your ugly face, sadly," he said, getting no more then a rose eyebrow from Severus.

"You should talk," Severus sneered, looking to the side with a smug look on his face. Sirius turned red, as James scowled. The other students turned to look at them with morbid curiosity; whenever James Potter acquired that look, he meant business. "Oh, is the mudblood's plaything mad?"

Within the blink of an eye, wands were drawn and hexes were being shouted and aimed at each other. Severus couldn't have hoped for a better opening, having an excuse if his Head of House didn't protect him from the school's wrath.

From the corner of his eye he could see the outline of someone turning the corner, and he knew he could not wait any longer. Severus twisted around, scrunching his face in concentration on the Gryffindor, and muttered, "Sectumsempra."

James' face contorted in pain, as his robes split open across his chest and abdomen, glistening slashes appearing on his skin. Blood seeped out of them steadily, as he fell to the stone floor with a loud thud. The resounding gasp told him all he needed to know. Severus straightened up, and watched as Remus and Sirius rushed to James, one of them looking worriedly at the downed boy and the other in an inner argument whether to do the same or glare at Severus.

Sirius wasn't given a chance to decide. The teacher that had been coming closer was the Potions teacher, and he started into a run toward them, pulling his wand out. "Mobilicorpus," he said, "All of you, get into the classroom! And you are to stay silent until I get back!"

Waiting until both bodies were out of the dungeon, Sirius and Remus turned to Severus, who glanced at them easily. "What did you do?" they asked harshly.

"Like I should tell you?" the smaller boy replied just as harshly, "That is for me to know, and you not to find out." Before anyone could stop him, he took up his bag and strode into the classroom, smirking all the way.

- - -

Added note (August 9, 2005) – This is mostly in reply to a review this got, since it got me wondering and I went on my own search in HPB and OotP. I also corrected my spelling of Sectumsempra, since it was pointed out I had it wrong by two letters or so.

It was stated the Sectumsempra spell couldn't have been made in 5th year, because James used a levitation spell on him during the OWLS, which was possibly the Levicorpus spell Severus created. No incantation was given in OotP when James cast the spell, and it is unlikely, though not impossible, that is was Severus' version of Leviosa, considering it is a nonverbal spell and that it is doubtful he would use as a verbal spell or give it out to other students. This is what muddles up the scene that mentioned that one of his spells was 'used against him, ironically.' However, I never said what spell was used against him. It could have been the toenail growth spell, or one we never heard of in the 6th book…or maybe, just maybe, Levicorpus, which I'll bring up a minute in more detail.

Another note is in that scene in OotP, Snape uses a nonverbal spell – or the incantation wasn't given to us, at least, if it was a verbal spell – that had the same effect as Sectumsempra (a gash on James' face, blood splattering, etc). The scene in OotP suggests he created the Sectumsempra spell in fifth year or below, while it gives few to no hints on when his Levicorpus spell was made, besides that James' spell incantation was never given to us.

Levicorpus. Going on the 'what if James got his hands on this spell,' this is when I have to ask myself "would he wait to use it?" In my mind, the answer is probably not. If I had found a spell my rival/enemy created and could make them miserable with, I surely wouldn't wait, I'd take the first chance I got to use it. And James seems like the type to do just that, in the Pensieve memory. So if he did get a hold of it by chance, it'd probably have been used one or twice before the incident in the books, if not by him then by Sirius. It is still unlikely to be this one, because they would have either needed to get a hold of the Potions book and look through it, or to have read Severus' mind to get the right word. There is the highly unlikely third possibility that someone leaked to them what the spell was, but as most of his group became death eaters, its unlikely that they would have went to James and Sirius with such a spell.

I'll quit now, before my mock-essay on the topic gets as too much longer!