Disclaimer: Harry Potter and the associated characters do not belong to me as I am not J.K. Rowling.

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Sectumsempra

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He is fifteen when he first creates the spell and his first and intended victim was James Potter. The deep gash in Potter's cheek had been vaguely satisfying until once again he'd been wrenched up by his ankles- by his own curse. He made a mental note to improve the power of the spell in that moment.

He had perfected it by the beginning of Seventh Year, having practiced on the mice that used to inhabit his house during summer. He never used the spell whilst at Hogwarts again, positive it would get him expelled if he picked on the new reformed Head Boy. But he knew he would get his opportunity. There was a war- he had picked his side and Potter had picked his.

The next human who felt the effect of Sectumsempra was a Death Eater-wannabe, much like himself. The Dark Lord wished for him to demonstrate the new spell and he gladly obliged, picturing the Gryffindor's face as he cast the curse. He gasped in horror when he saw just how bad the damage was though and felt a wave of doubt. Could he really do this again? Voldemort wanted to time how long it took for the man to bleed out. He was dead within twenty minutes.

He vowed in that moment to never use the curse again… that was until news of the Potter-Evans wedding spread through the Wizarding world. Then he knew- he could use it- a one off- on the person it was originally intended for. The one person he thought he could happily watch die. The person who had stolen the one thing he had ever cared about. Deep down he knew it won't make Lily love him, but his logical side wasn't functioning and all he can think of is Potter writhing in agony; Potter being punished; Potter begging for mercy; Potter feeling an ounce of the pain he has had to deal with for the past few years. So when the call comes that the Order has broken into one of their bases, he sneaks off from the battle and goes to find his arch-enemy and make him pay.

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James isn't quite sure how he managed to get separated from the rest of the Order- from Lily- but he is by himself running through the dark corridors of a known Death Eater base. Probably not the best situation he'd ever found himself in.

As he turned left at the end of the corridor, he noticed a very distinctive painting of Grindelwald.
"Shit!" He swore, realising he had gone around in a big circle. Navigation had never been his strong point. He headed back on himself, trying to find any form of life in the stone-cold silence.

"Potter." A voice growled. Ok- maybe not just any kind of life. James turned around as casually as possible, drawing his wand to see Severus Snape, the stark white of his face contrasting greatly with the black of his robes; hair and surroundings.

"Ah, Snivellus- it's been too long." He drawled. "Lost your Death Eater mask I see? Shame- sure it would've been an improvement." He glanced slightly at Snape's extended wand, back to his face- shocked to see a ghost of a smirk there. Snape flicked his wand casually, James barely caught the movement and only just had enough time to cast a shield charm.

"Is that really the best you can do, Potter? Still fooling around with childish pranks and insults?"

"At least I have an ounce of humanity and I don't have to kiss Voldy's arse on a daily basis. Face it, Snape- you're his bitch."

Snape sneered, sending another curse James's way. He blocked it just in time before sending one back at Snape.

"I always knew you'd go dark side, Snape." James panted, dodging as Snape sent yet another spell his way.
"I always knew you'd be foolish enough to join Dumbledore's cause- arrogant enough to believe you'll win!

As if to prove his point, his next curse caught James' left arm, leaving scalding burns. James hissed before sending another shot back to Snape. He missed, but managed to dodge the next curse that was aimed at him.

"I'm fighting for what's right." James spat. "And my arrogance was always usually well-founded." He managed to smirk.

"You'll be sorely disappointed, Potter. This time next year the Dark Lord will be Minister and the world will be rid of the filth that is muggles and mudbloods!"

James' eyes flashed and his next shot hit Snape in his chest, sending him flying back a few feet. He stood up and dusted himself off, clutching his chest with one hand, noting James visibly shaking with anger.

"Hit a nerve, did I Potter?" he wheezed, something like triumph in his eyes "How is your girlfriend these days anyway? Avery sends his l-"

But he was cut off by a violent blue light that snapped his wand arm in half. He screamed, but quickly swapped hands. The duel turned fierce and bloody, one curse wouldn't have even hit before another was cast. Snape soon shattered James' glasses. James became extremely vulnerable then, not being able to see well enough to aim properly or dodge effectively.
He was a walking target and soon enough he was going to get hit with something deadly. James noticed a blue light coming at him in time to miss it and stumbled, just enough to make him completely helpless.
Snape grinned maniacally- his moment had come!
"SECTUMSEMPRA!"

"NO!"

That voice…

Both James and Snape had registered Snape raising his wand before a flash of red whizzed towards his victim and then an entirely different flash of red ran in front of him from the left and that shout...no.

"Lily!" James registered as the spell hit her full in the chest and in time to catch her as she crumpled, turning her around in his arms.
He quickly took the chance to repair his glasses, just in time to see Lily convulsing in pain.
He fell to the floor with her and gasped aloud as he saw the blood spreading across her chest and abdomen. Her eyes rolled back into her head.

"SNAPE!" he all but screamed at the man who was frozen at the other end of the corridor. "The counter-curse!"

He didn't move. No no no! He thought. Not her.

"Snape!" James yelled frantically, holding Lily's face in his hand.
"Lily! Lily? Stay with me, darling."

Her mouth quirked as her head lolled in his arms.

"Darling?" she scoffed, coughing slightly. He raised his wand shakily, racking his brains to try to think of a suitable healing spell.

"Ok...ok... Teleo-"

"No." Came a sharp command from the end of the corridor.

"What?" James demanded.

"That will make it worse." Snape explained, moving quickly to Lily's side.

"The counter curse then!" James cried.

Snape merely shook his head, his face white. "I haven't got one yet."

James blanched. "You've got to do something! She's dying Snape!"

Black eyes met hazel and in that moment both understood that the death of this red-head was the last thing either ever wanted to happen. They needed to work together.

"Follow me." Snape said, standing briskly. "There's a potion that might work."

James hesitated, but then knew he had to put his trust in the only person who could help Lily right now.
He scooped her up in his arms; her blood soaking through his shirt and ran after Snape. As soon as they hit some form of potions room, James laid lily down on a table top and Snape went to work.

"Keep her talking." Snape commanded.

James heard him. "Lily...Lil..." he gently shook her "Lily?"

Her eyelids fluttered open.
"James..." she sighed, her face breaking into a smile. "You're here." she sounded delirious.

"Of course I'm here." he said in a strained voice, taking her hand. "By your side is my favourite place to be."

"I thought it was on top of me?" Lily chuckled, before shuddering. James stroked her hair.

"Anywhere around you..." he murmured. "Adjacent, opposite, on top of you... hovering a little to the left...just so long as you're there." he kissed her hand.

"You're s-so cheesy potter." she smiled, and then shuddered again.

"Why did you jump in front of me like that Lily?" James asked- his voice pained and quiet.

"Bec-cause I love you." she rasped, as if it was the most obvious reason in the world.

Tears formed in James' eyes. He kissed her forehead, closing his eyes as he did so and then rested his forehead against hers.

"I love you too." he whispered. "Promise me you'll stay here with me?"

She didn't appear to hear him though as she shivered again.

"I'm so cold." James automatically moved to cover her body with his own as much as possible and pressed a kiss to her nose.

"Don't worry...you'll be better soon..." he soothed.

"I can't feel... anything." Lily said quietly.

"Snape! Hurry up!" James hissed.

"I'm doing my best Potter!"

"Snape!" Lily asked fearfully, trying to move before James pushed her back to the table top. It was no secret to her what side Snape had chosen now.

"Ssh.. lily. I won't let him hurt you. He's just got some medicine to make your cuts better."

"Promise you won't let him touch me?" she whispered tearfully.

"Promise. You'll be safe, I'm here. I won't let anything happen to you." he cupped her cheek gently.

"G-good. You always protect me." she closed her eyes. "James...I'm sooo tired..."

"Just stay with me a bit longer, Lil. Then you can sleep as long as you like."

"I c-can't help it...everything is so...so fuzzy..." her eyelids drooped.

"Lily? Lily! Wake up!" he shook her as she fell unconscious. "Snape where the fuck is that potion?"

Snape returned to the table then, holding a pot filled with a steaming paste. He took a spatula and held it in his hand, putting down the pot and then reached for the buttons on Lily's shirt, but not before James slapped them away.

"Don't you touch her." he bit out. "I'll do it."

He gently opened Lily's shirt, revealing five deep gashes that were still oozing blood.
Snape picked up the pot.

"Not on your life, Snape." James warned, holding out his hand.
Snape's lip curled before dutifully handing it over and allowing James to apply the paste. The effect was almost instantaneous. The skin began to knit itself together again and the bleeding stopped fully. James almost cried in relief as he felt for Lily's pulse and found it weak, but steady.

"She'll sleep now." Snape said, a barely noticeable tremor to his voice.

James nodded "I'll get her to St. Mungo's." he said, whilst scooping her into his arms again.

"You actually love her," Snape spoke. James paused at the door. "and she loves you." He laughed bitterly. "You don't deserve her, you know. You don't deserve to have her die for you." There was a brief pause.

"I-I know." James said gruffly, his voice heartbroken.

"How does it feel? To be unworthy of her love? Knowing that she could leave you at any point?" James cleared his throat and turned to look at Snape.

"I can imagine you know exactly what that feels like."

Snape's eyes narrowed.

"Y-you don't want her dead, Snape." James whispered. "She was probably the only true friend you ever had and you threw it away. Had that curse hit me, I-I'd be dead right now- don't think I don't know that…but her… You care about her. There's no other way I would have trusted her safety with you tonight.
"but... That's where the similarities between you and I end. You made her leave a-and then..." he shuddered. "You may have saved her tonight... But nothing will ever redeem you from what you put her through." James said quietly. "Even more so because you do care about her. How could you even think about hurting her? If it were down to me, you'd be hanging by your balls in Azkaban for the rest of your miserable life" He swallowed then looked to Snape.
"Sh-she's done with you too. You heard her. She abhors the idea of you even touching her. You destroyed her trust- you sided with the people who are trying to wipe out muggle-borns, people who are trying to kill her." He took a steadying breath and stared at Snape "I hate you so much. Detest the very ground you walk on. I don't think you can understand the loathing I feel for you for what you are and how you've hurt her. Whatever you feel for me, I swear I hate you ten times more. And I may not be worthy of her but I do my bloody best every day to do right by her- to protect her and keep her happy and safe. I would never, ever hurt her. Unlike you..." he turned back to the door "I hope you're proud of yourself. Enjoy being a Death Eater. The dark arts are all you've ever cared about after all."

He made to walk out but stopped as he heard a clearing of a throat.

"Potter," Snape said croakily "for whatever reason she's chosen you. Look after her."

James didn't turn back. "I will." he said with such conviction and truth there was no doubt he would.

The third victim of Sectumsempra was intended for Potter- and he did get a Potter, just not the right one. Seeing Lily almost bleed to death prevented him from using the curse for another eighteen years. Yet again he hit the wrong person and he was hit with flashbacks of that fateful night. The night it came crashing down around him that killing her husband would hurt her more than he could handle and so from then on he did not want James Potter dead; the night he first felt doubt about what he was doing because he saw her actually get hurt by the war- and how could any world be better than one where she was living?; the night he started to feel sick at the idea of hurting other human beings.

It was the beginning of what would lead Severus Snape to save the world- and all because of one spell- one deadly, horrific, powerful spell that a fifteen-year-old boy with idealistic views on power created…and it would ultimately lead to his destruction- and indirectly his salvation.

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A/N – Thank you for reading. This was inspired by a short drabble I read once where Snape goes to kill James and Lily jumps in front. I can't remember who it belonged to so thanks to whoever it was