Boggarts.
Sherlock's never thought of them much. He knew the textbook definition of them (of course he did; it was ALWAYS important to protect oneself, as he'd learned earlier in life). And After Professor Lupin announced that they were to be doing a lesson on them next period, of course he'd read up and lectured John on them.
The only problem: Sherlock, for once in his damn life, had no idea what was going to happen.
Sherlock fancied himself fearless; of course he'd had the occasional bout of nerves when it came to a situation, but he'd never really, truly felt terror. So he wasn't really worried when the Boggart lesson rolled around. He was sure that it would be something just a bit distressing, like finding his brother dead or not being able to solve a mystery.
It didn't mean that deep, deep down in his heart; he was just a bit worried of what he might find.
~.~
Professor Lupin smiled grimly at the class as they filed in that day.
After lecturing them about the boggarts and how to fight them, Sherlock felt a small thrill of excitement after Lupin opened the cupboard. Beside him, John flinched and grabbed Sherlock's arm almost subconsciously. "Sherlock," he muttered breathlessly, "aren't you just a bit frightened?" Sherlock slipped on his trademark smirk. "John, it's an illusion. How could you possibly be scared of it?" John sighed. "You are such a help sometimes," John gritted his teeth.
Sherlock watched passively as the third year Ravenclaws and Hufflepuffs walked up to face their deepest fear. Some were stupid; a giant towering tree? Really? Do better than that, Donovan—and some were just a tad dark, such as Irene Adler's fear of her father. Still, Sherlock remained unimpressed, and when his name was called out, he marched up confidently. Perhaps it would just be Mycroft finally going on a diet, he thought sarcastically.
The boggart morphed immediately from a huge cockroach (really Anderson?) to…a shortish well built thirteen year old with a round face that oddly resembled a hedgehog. "John?" Sherlock said, for once actually, truly confused. John smirked—a facial expression that hardly fit John Watson at all. It looked alien and wrong on that face.
"Sherlock Holmes," he said, his lip curling. His voice sounded wily and nasally, a bit like Jim Moriarty's—a second year Slytherin whom Sherlock disliked from the start. "Such an apathetic, heartless know-it-all—why did I ever waste my time on you?" he sneered. "You're pathetic. Absolutely pathetic. Worthy of death." His expression changed. It was now full of bitter, terrible glee, much like Voldemort's when he made a kill. It was very disconcerting to see it worn so perfectly on his best friend's face. Sherlock felt a pang of something—it felt painful, but worse, as if someone had torn out his insides and twisted them into a giant knot. Sherlock fell to his knees.
John's black grin grew wider. "Death. You should die. You should die by my hand—or better yet, watch the only person you've ever cared about die." Sherlock couldn't think. He couldn't think, couldn't speak—he was out of words. Sherlock Holmes was speechless. He felt something wet congeal in his eyes and drip down his face. Drip drip drip. Down these tears, borne out of pure terror, fell. He watched through blinded eyes as John pull out a knife.
An animalistic noise erupted from the back of the class. John Watson, third-year Hufflepuff, careened forward out of the silent class and grabbed Sherlock. "Sherlock," he pleaded. "Sherlock," the Boggart mimicked. John paid no heed. "Sherlock it isn't real—Sherlock listen to me goddammit. It isn't real. I will NEVER leave you. Never. Goddammit, you ass, just do the spell already. You know it isn't me. You know that I will never, ever do this."
Sherlock shook his head. His face was still painted with salty tears. Huh. He'd never cried before. He pointed his wand upwards. "R-r-riddikulus," he said. The Boggart-John Watson disappeared.
But the real one didn't. John never disappeared. John never left. And Sherlock knew that he wouldn't ever.
I know it was terribly out of character, but I just thought that Sherlock cared just as much, maybe more about John as much as Sherlock cared about John (demonstrated in the Reichenbach Fall, damn you Moffat) and wanted to prove it. R&R, please.