A/N: I wrote this on Halloween when everyone was distraught over the anniversary of Lily and James' death. Enjoy! Or don't, this is pretty morbid.
Mulciber was already in jail by the time they brought in Sirius Black.
He'd been there for a good week when he felt the all too familiar drain that came with the guards. This time was different, though. Either he was getting weaker, or there were more of them than usual. He shuddered and forced himself up against the cold iron bars of his cell, resting against them as comfortably as he could while keeping a clear view of the corridor. The few seconds that passed felt like hours. Mulciber's sanity faded and life grew more and more dismal until, finally, they rounded the corner.
There were at least eight of them, all huddled together 'round the newest inmate. Mulciber started and sat up a bit straighter. It had only taken four guards to subdue him when he'd been brought in. A glimmer of hope shone in the middle of the huddle, and he wondered, very briefly, if the rumors weren't true, if his master wasn't really dead after all, but instead captured. It'd be just like the Ministry to put forth such a lie to calm to public. But if the Dark Lord was here, on the very same corridor he was sentenced to-
His hopes were short lived, but so was the disappointed drop of his stomach. The sight of Sirius Black inside the walls of Azkaban was the first material thing that had been able to make him smile in weeks. As the guards swept their way to the ends of the corridor, Mulciber found the strength to prop himself up on his knees, clutching the bars tightly in his hands.
"Black." There was no answer. "Black." He beat his chains against the bars. "OI, BLACK!" The only sound was the sifting of chains in the distant cell. "I know what they're saying about you. It's wrong. All of it's wrong, isn't it?" He inched closer to the corner of the cell, willing himself to be as close to Black as possible. "You're not the big man they all think you are, are you? My master…would never…lower himself to the likes of you." The silence grew more frustrating with every passing second. "I know what you really are," Mulciber accused, his voice rising. "You're filth. You're just like the rest of them. YOU'RE A FILTHY BLOOD TRAITOR, A DISGRACE." He paused to take a breath. "But you-you're nothing like us, you sick excuse for a Wizard. You're nothing like me."
At long last, there was a reaction. The shadow of Sirius Black's face appeared in the dim light of the prison, already weathered and worn, aged far beyond the face of a twenty-one year old. Mulciber was taken aback. This was not the face he had known during his days at school.
"You're right," Sirius said. "I'm nothing like you." He slunk back into the confines of his cell, leaving Mulciber as starved for company as he had been a mere hour before.
As Mulciber fell back onto the cold stone floor of his cramped cell, he realized that he was going mad. He laughed. He laughed at everything he had once been, everything he had hoped to become, everything he amounted to now, which wasn't much. Through the echo of his laughter against the stone walls, Mulciber heard a second voice laughing with him. He grinned and a fresh wave of laughter overtook him.