The person who came was none other than Sirius Black.
He patted the overjoyed Harry on the shoulder, but his gaze went past the crowd directly to William, who was standing behind the podium, "Remus said you didn't have any more classes after this."
William raised an eyebrow, looking at Sirius's face which was a mix of exhaustion and excitement, and guessed a bit.
He nodded, "Yes, let's talk in the Room of Requirement... and then." William raised his voice, "You two Mr. Weasleys, retract your left foot from the doorway, Mr. Filch will be here any moment and I'd like this classroom to be back to its original state by the time I return, or else..."
The twins turned back in silence, looking at the completely unrecognizable classroom, feeling rare regret...
"I already told you! We shouldn't have used such a large dose of—" George began to blame Fred.
Elsewhere, ten minutes later, the air in the Room of Requirement was warm and comfortable, a great fire burned in the fireplace, and Kabuda was snugly curled up on the couch's armrest, dozing with a shiny Golden Galleon in his arms, while Dobby was quietly busy at the tea table in the corner, preparing hot tea for the two people seated on the couch on either side.
"It looks like, you found him?"
William took the steaming cup of red tea handed by Dobby but did not drink it, instead, diving straight to the point.
"Found him!"
Sirius drained a large mouthful of tea with a long sigh, "It was like a nightmare. I searched through half of the United Kingdom, every black market, and possible place where a house-elf might gather or be sold—… finally, can you guess where I found him?"
"Where?"
"In a wizard bar in Edinburgh, wearing chains and working as a waiter—"
Sirius sighed, "Those people who broke into the Black Mansion, they obviously took Kreacher as a masterless elf and sold him off—but Kreacher has a master, they nearly drove him insane trying to make him change allegiance…"
"So, how is he now?"
"His body is mostly unharmed, but his mind… it's very, very damaged."
Sirius frowned, showing a rare complex emotion towards the house-elf he had long despised, "More deranged than before, even after I bought him he couldn't recognize me, always screaming, shouting, banging his head against the wall… I had no choice but to use a spell to calm him, but it's no solution."
As Sirius spoke, he reached under his cloak, pulling out a patched cloth bag.
He paused, looking at William, "I don't know what to do, but… maybe Regulus can?"
"After all, compared to me, he's more of this fellow's master."
The man's expression was somewhat complicated, after all this time he clearly was still not used to how to face his brother who had changed so much from the one he remembered.
William nodded, "Regulus... he's now a Slytherin ghost, Dobby."
With a "pop," Dobby appeared in front of William, his large eyes fixed on him, "Yes, sir, what are your orders?"
"Please go invite Mr. Regulus Black, and tell him that he might be frequenting the Slytherin dungeon or the clock tower—if you don't see him, you can also ask Tom."
"Okay, sir!"
Dobby nodded and vanished again with a disillusionment pop.
Then came a long silence, finally, after who knows how long, the door to the Room of Requirement slid open silently, and Regulus Black, a semi-transparent ghost clad in a disheveled wizard robe, floated inside, his gaze first landing on Sirius, carrying a hint of detachment and scrutiny.
"Sirius."
He greeted calmly.
"Regulus."
Sirius's voice was somewhat dry, and he only greeted back, seeming like he wanted to say something but ended up not speaking.
Regulus then turned his gaze to William, nodding slightly, "Professor Richard." And then, as if sensing something, his sight fell to Sirius's feet—on the patched cloth bag.
So, no one spoke anymore, Sirius bent down, untying the tightly knotted cloth bag, revealing the inside.
It was Kreacher, he looked even more petite and shriveled than the last time William saw him, his enormous fleshy nose buried in his excessively gapping knees, his body, even though charmed into a slumber, trembled uncontrollably, a muffled, broken whimper escaping his throat from time to time.
On Regulus's face, the usual calm was finally broken, he floated closer, a clearly visible heartache appearing on his transparent face.
"Kreacher?"
His voice was soft, yet it seemed to carry some sort of magic.
Without even waiting for Sirius to lift the spell, the curled-up house-elf shuddered abruptly, the whimpering stopped as well, he extremely slowly, as if using all his strength, lifted his head, and when his tennis ball-like eyes, full of bloodshot and discharge, saw the ghost hovering before him, everything seemed frozen.
At that moment, Kreacher's mouth opened wide, a strangled sound emerged from his throat as if being throttled, after a few suffocatingly silent seconds, a gut-wrenching wail mixed with endless grievances, longing, and pain burst forth, "Master—!!"
The elderly elf lunged forward, trying to hug Regulus's legs but instead passed through his ethereal body, causing him to fall to the ground, yet that didn't stop Kreacher, he prostrated on the floor, banging his head on the floorboards with a dull thud, his sobs grew piercingly heart-wrenching.
"Kreacher failed, Kreacher didn't fulfill the master's orders! Kreacher let Master Regulus down! Kreacher even couldn't watch the house! Kreacher is a bad elf! A bad elf! Master Regulus... Kreacher misses you... Kreacher followed your orders... always always... waiting..."
The elf's words became more fragmented, until in the end they barely formed sentences—clearly, Kreacher had deeply remembered Regulus's last command to him in his heart, and when he saw the other again, those memories drowned him like a tide, completely scattering the little elf's mind.
Regulus's ghost knelt, trying to soothe Kreacher with his hand, although he couldn't really touch him.
"Alright, Kreacher, alright…"
Regulus's voice was exceedingly gentle, bearing a warmth rare even when he was alive, "Don't worry, you didn't fail, Voldemort has been defeated, the Black Mansion has also been reclaimed, you did well Kreacher, you fulfilled my command excellently—
"You lived well."