Hello, Luke here. Just to mention, I'll be keeping Moran as I started writing him despite the canon version seen in Series 3, hope you're all okay with that and thanks for reading! Now onto the reason you're here:

John had been right: by the next morning both Harry and Ron were regarding the encounter with the creature on the third floor as a great adventure. When the three went down to breakfast, chattering in hushed voices about the night before, they found Sherlock already sat reading a tatty leather-bound book on the Gryffindor table. Ron made a hasty and slightly awkward apology for being so short with the Ravenclaw boy the night before.
"It's not you we should be mad at anyway," spoke up Harry whilst chewing a jammy mouthful of toast, "Malfoy was the one who got us out there, I bet he tipped Filch off too."
There was a general murmur of agreement. John glanced over to the Slytherin table, and sure enough Malfoy was staring at the group of them with a mixture of confusion and anger.
"I wonder what it must be like in his funny little brain." Sherlock said, following John's line of sight and caught Malfoy's eye in time to wave jauntily and give a charismatic wink. The blonde boy's pale cheeks flushed pink with indignation and he flung out his hand, John supposed, to reach for either Crabbe or Goyle. However the skinny shoulder Malfoy found in his grasp was neither.

James Moriarty lifted his gaze from his breakfast to where Malfoy was gripping his cloak with eyes as cold as winter and a sly reptilian smile. As John watched, Malfoy hastily pulled his hand away, the pink in his cheeks darkening and spreading to the tip of his nose, but Moriarty seemed to giggle and lean in to say something. A few moments passed in which the Slytherin boys exchanged a few words until they simultaneously lifted their gaze to where John was sitting, matching smirks stretched across both their pale faces. A chill ran down John's spine and he averted his gaze quickly back to his own table.
"Did you see that?" he whispered, leaning over to Sherlock whilst keeping his eyes firmly fixed on his plate.
"Hm? See what?" Sherlock had turned his attention back to his book. John warily looked back to the Slytherin table only to find that both Malfoy and Moriarty were gone. He craned his neck to search the Great Hall but couldn't spot them anywhere.
"Nothing," he said, ignoring the unease that had begun to spread in his stomach, "don't worry about it."

- - - - - - -

"This is bullshit," came the rough murmur in John's ear in one particularly boring Defence Against the Dark Arts class. Sebastian Moran seemed as if he was in physical pain as Professor Quirrell stammered over his words and looked positively terrified by the idea of the creatures he was explaining.
"Tell me about it," John sighed in reply. He had hoped (like Seb) that this class would have been a more hands-on one and had been itching to try out some real combat magic. The reality of books, lectures and note taking was quite a disappointment.
"Where's your mate? The smart one?" Sebastian asked whilst doodling a picture of a tiger on the corner of his parchment. John shrugged: it wasn't unlike Sherlock to miss lessons, the boy already knew everything he needed to and had managed to reduce Quirrell to tears in his second lesson by deducing his past experience with a Romanian vampire. Quirrell didn't seem to mind his absence.

John hazily watched Sebastian draw a large magpie to fight the tiger whilst the rest of the class listened disinterestedly, raising their heads every so often to take the occasional lazy note. Sebastian edged his wand out of his pocket and pointed it at his parchment, muttering a quick spell. John gasped quietly as the creatures writhed and came to life on the page, swiping at each other fiercely. Sebastian gave a shy grin, obviously rather proud of himself, before suddenly cursing violently under his breath and splotching his ink, destroying his fighting animals. Something small and white had bounced off of his face and was currently fluttering haphazardly around the two of them.
John recognised it as Sherlock's handiwork immediately and swiped the paper bird out of the air before the other students noticed. Spreading out the note he saw it said simply in the distinctive untidy scrawl: 30 seconds, The Black Lake. He showed it Sebastian who seemed to realise something John didn't and began silently shoving his books back in his bag. John followed his lead instinctively and was just snatching up his quill when a small dark object whizzed into the room through the open window and land neatly on Quirrell's desk. There was a moment of stillness where the Professor, along with the class, leaned in to look at the little shape. Only then did it explode in a foul cloud of stinking smoke which filled the room with fumes and darkness.

John and Sebastian raced past their spluttering classmates, holding their breath until they burst into the corridor and rushed out towards the grounds. When they finally got there, hooting with laughter, they found Sherlock sat on the grass beside the Lake with a small smile playing across his lips.
"Levitation charm," he called gesturing up to the building and the open window before John could ask. Sebastian squinted up at the window and gave a long impressed whistle, "Not bad, mate."
Sherlock shrugged but Sebastian gave him a once over before offering the dark haired boy his hand and pulled him to his feet.
"So what's the plan? I don't recon you pulled us out of lesson just so we could enjoy the view," Sebastian's tone was friendly and his lips were spread in an accommodating smile but John recognised the challenge in his eyes. John had seen the steely glint himself in his last flying lesson and it had resulted in a breakneck speed race in which they both nearly fell off their broom (then again a night in the Hospital Wing would've been a preferable result compared to the detention dished out to them by Madame Hooch). Sherlock looked over his shoulder, towards the Forbidden Forest.
"I thought the two of you might want to try a hand at some real Defence Against the Dark Arts," he said, turning back to the other boys with a grin. The sensible part of John told him that this could only end badly. However, all the other parts of John were jumping with excitement and he nodded fiercely as Sebastian whooped "About bloody time!" and gave Sherlock a friendly thump on the back which nearly toppled his skinny body.

As they set off towards the forest Sherlock began speaking in fast fluid waves of deductions that he'd made about the grounds and the evidence that something obviously very large had crossed over here and that he couldn't understand how no one had noticed the change in the pattern of the dew on the grass every couple of feet; and couldn't anyone notice that smell? And look at the behaviour of the owls and wild-birds that flew overhead and how they all avoided that area of sky above the treeline as if something predatory was lurking amongst the woodland. They approached the treeline as he was just explaining the finer details of the movements of the centaurs and how it couldn't be to do with them, especially with Mars this close to Jupiter, when John flung up a hand and stopped in his tracks.

Something had moved, too deep in the forest for John to have seen it clearly, but amongst the shadows of the trees the darkness had shifted.
"John, what is it?" Sherlock asked, his voice rising in pitch with excitement. John drew his wand slowly and began to creep forward.
"There's something in there," he murmured as the three of them crossed the gloomy threshold, wands raised. "I dunno, there was something large, deep in there and it-"
They heard it this time, the rustling of dead leaves. Sebastian and John turned and bravely aimed their wands towards where the noise had come from, however Sherlock gave a small "Aha!" and ran straight towards it. The other two bolted after the reckless Ravenclaw, protesting uselessly. John's heart was in his throat as he watched the blue scarf and dark robes melt into the blackness of the forest, his mind suddenly picturing horrific monsters stumbling out of the shadows and swiping at his careless friend.
"Sherlock! Come back!" he yelled with bated breath as he swerved through trees and over logs in the greyish half-light, Sebastian right behind him. John had decided to ignore the fact that they were making enough noise to lead whatever hid among the shadows right to them and poured on an extra spurt of speed.
"Sherlock?!" John skidded to a halt, trying to hear over his own blood pounding in his ears. He turned to Sebastian, "Which way?"
The other boy paused, set his jaw and steadied his breathing, cocking his head to one side and listening intently. Nothing seemed to move. There was no sound of Sherlock's light footsteps racing over dry leaves. There wasn't even the natural sounds of bird or the wind through the branches. It was if the forest had died and they we're unwelcome trespassers on a private funeral. Somewhere in there Sherlock was alone and he was in danger. John felt the usual queasy feeling of unease in his stomach harden like iron in his gut and he grit his teeth, gripping his wand tighter. Sherlock needed him. Sebastian's eyes flung open suddenly, and John had heard it too: a distinctive shout that echoed around in the dead air. Sebastian pointed to the West, "Over there!"
As they ran the shouts became clearer and the fear in John's chest that Sherlock was hurt quickly diminished when he realised that Sherlock was actually arguing, just very loudly.

They burst onto the scene and it took a second for John to realise that his best friend wasn't in fact engaged in a heated debate with some monstrous creature from the forest's depths, but with two older boys. Judging by their prefect badges John guessed they were at least four years older than themselves, but that didn't seem to bother Sherlock.
"….you're the one who was in here in the first place, there's no point in trying to gloss over that. Just because you practically run the school doesn't mean you can go into restricted areas for such high priority purposes," Sherlock snarled viciously at the taller of the boys who wore a Slytherin tie and a grim expression.
"Sherlock, we are on prefect business, I suggest you take yourself and your little friends back to the castle before I exclude all three of you," came the icy reply. For all the coldness and composure that the Slytherin prefect wielded, the Gryffindor prefect behind him blushed violently and tried to hastily correct his loose tie and untucked shirt. Sherlock turned to acknowledge both John and Sebastian who were stood there panting, both trying to figure out what was going on.
"John, Sebastian, please let me introduce to you my brother, and his associate, Lestrade."
Mycroft Holmes glared glacially at the both of them but to their credit neither of the Gryffindor boys retreated. Lestrade ran a hand through his hair nervously before reaching out and tapping the bark of a tree with a knuckle, "Yeah, all looks ship-shape. Prefect approval all round. Right. Let's go."
"Wait," Sherlock snapped, "What do you know about the forbidden corridor on the third floor?"
Mycroft's eyes went from winter-chill to freezing blizzard in less than a second. Before Sherlock could react there was a wand being pressed underneath his chin.

"Stay away from that corridor, Sherlock. That's an order." And with that both prefects were gone, sweeping back to the castle and disappearing into the forest gloom. As John looked up to the canopy of leaves he could see the sunlight beginning to fade, "We better get back, I don't fancy being here in the dark."
Sebastian nodded in weary agreement and started to trudge back the way they came whilst Sherlock stomped moodily behind and John kept a wary look out.
"Nice family you got there, loads of brotherly love." John murmured before muttering "Lumos" and holding out his wand like a torch as the forest grew rapidly darker. Sherlock growled slightly in response and said nothing more until they finally reached the edge of the treeline. The moon illuminated the grounds with a ghostly silver light and John extinguished his wand. Sebastian cracked his knuckles noisily, turned and grinned at the other two boys.
"Not all is wasted at least," he snickered, "at least you've got some juicy gossip on your brother's private life."
Sherlock hummed in approval of the suggestion, "Yes." A wicked grin snarled over his lips, "and I wonder what he's willing to tell me in exchange for silence."