**A Discovery of Unsettling Proportions**

Sirius Black was sauntering down the dimly lit corridors of Hogwarts, whistling a carefree tune as he made his way back to Gryffindor Tower. He had just finished a late-night study session in the library and was looking forward to collapsing onto his bed with a good book. As he turned a corner, he stumbled upon a sight that made his eyes widen in shock. There, casually strolling around the perimeter of Barty and Evan's dormitory, was Regulus Black, his younger brother. But it was not the fact that Regulus was out of bed at such a late hour that caught Sirius's attention - it was the state of his attire. Regulus was wearing nothing but a thin, black thong and a faded white t-shirt, his dark hair tousled from sleep. Sirius's jaw dropped as he took in the sight, his mind struggling to process what he was seeing. What on earth was Regulus thinking, walking around in such a getup? And why, in the dead of night, was he parading around Barty and Evan's dorm, no less? Sirius's eyes scanned the area, half-expecting to see a group of snickering students or a bewildered Barty and Evan, but he was alone. The corridors were deserted, and the only sound was the distant creaking of the castle's ancient wooden floorboards. Without hesitation, Sirius strode towards Regulus, his long strides eating up the distance between them. "Regulus, what in the name of Merlin's beard are you doing?" he hissed, trying to keep his voice low. Regulus, oblivious to his brother's approach, continued his leisurely stroll, humming a tuneless melody. It wasn't until Sirius cleared his throat that he spun around, a look of nonchalance plastered on his face. "Oh, Sirius. What's up?" he drawled, eyeing his brother with a lazy gaze. Sirius's eyes narrowed, his temper rising. "You know exactly what's up, Regulus. What's going on here?" Regulus shrugged, a mischievous glint dancing in his eyes. "Just couldn't sleep, I suppose. Took a walk." Sirius's incredulity was written all over his face. "In...that? You expect me to believe you just happened to be walking around in your...undergarments?" Regulus's smirk faltered, and for a fleeting moment, Sirius saw a flash of guilt. But it was quickly replaced by a mask of defiance. "What's it to you, Sirius?" Sirius's patience had worn thin. With a swift movement, he reached out and grasped Regulus's wrist, pulling him towards him. "Come with me," he growled, his voice low and menacing. Regulus's eyes widened in protest, but Sirius was already towing him down the corridor, towards his own dormitory. "Sirius, let go! What are you-" The words were cut off as Sirius yanked Regulus into his room, slamming the door shut behind them. The sound of the lock clicking into place was followed by a loud, resounding smack, as Sirius's hand connected with Regulus's backside. Regulus yelped, his eyes wide with shock. "Sirius! What the-" "Don't you dare walk around Hogwarts in that state, Regulus," Sirius thundered, his face inches from his brother's. "You could get us both expelled!" The room fell silent, the only sound the heavy breathing of the two brothers. Regulus's face was reddening, his eyes welling up with tears. Sirius's chest heaved with exertion, his anger still simmering just below the surface. And then, in a voice barely above a whisper, Regulus spoke up. "You have no right, Sirius. None at all." Sirius's eyes locked onto his brother's, a spark of curiosity igniting within them. What did Regulus mean? And what secrets was he hiding beneath his carefree exterior? The game, as they say, was afoot.