Disclaimer: J.K. Rowling owns all characters, concepts, and locations. I am not making any money (or trying to make money) off of this story, it's just for fun!

Sirius Black shoved his dark, elbow-length matted hair out of his once- handsome black eyes, his anger nearing the point of frenzy. He paced the dirt-caked floor of a dingy tattered shack—the Shrieking Shack—in the village of Hogsmeade near Hogwarts School of Witchcraft and Wizardry. The Shrieking Shack was known in the wizarding world as the most haunted building in England, but Sirius knew the truth. The howls that had once come from this building over fifteen years earlier had come from the throat of a teenaged werewolf. Sirius, James Potter and Peter Pettigrew had witnessed Remus Lupin's transformation in their youth many times. But James was dead and Sirius had spent twelve years in Azkaban because the whole of the wizard community—including Remus—believed that Sirius's spying for Lord Voldemort had resulted in James' and his wife Lily's murders.

But it hadn't been him! Sirius pounded his weakened fists against a peeling wall of the shack. It had been Peter leaking information to Voldemort all along and he, Sirius, had believed it to be Remus, just as Remus still thought it had been him. Now though, now Sirius would take his revenge. Peter hadn't blown himself up twelve years ago, he was alive—Sirius had seen Peter perched on a redheaded boy's shoulder in a photograph in the Daily Prophet a few months earlier. Peter was alive and he would pay; pay for the wedge of suspicion he had driven between Sirius and Remus with his betrayal, for the twelve years of misery Sirius had spent in Azkaban with the dementors, for the lifetime James's son Harry had spent with Muggles, misunderstood and unloved. But most of all, Peter would pay for his cowardice, for handing Lily and James Potter—two of his best friends—over to the darkest of dark wizards in over a century, Lord Voldemort.

Sirius let out a howl of pain and frustration that reverberated off the bare walls and around the desolate room. "It's all my fault!" he croaked to the empty room, his voice hoarse from twelve years of near silence. "If I hadn't persuaded James to use Peter as the Secret Keeper, he and Lily would still be alive." He hung his head in shame, feeling like the worst of cowards. Abruptly, his head snapped up, sunken eyes blazing in his thin, vampire-like face. He would kill Peter tonight. It was the night of Halloween; the entire school would be at the feast in the Great Hall and Gryffindor Tower would be empty. If he could find that bandy-legged ginger cat, he would have an ally to play lookout for him. Either way, he was going in.

* * *

Sirius searched the school grounds from the edge of the Forbidden Forest, his sharp eyes and sharper ears alert. The shadows moved eerily across the grass, sending a shiver down his spine. A voice nearby made Sirius jump, and his furry black ears perked up. Before leaving the tunnel beneath the Whomping Willow, he had transformed into a big black dog. No one in the castle but Remus knew he was an illegal Animagus, and he was hoping against hope that his friend would have been too ashamed of betraying Dumbledore's trust as a youth to reveal what he had allowed his friends to do for him. The voice faded and Sirius could make out the huge figure of Hagrid trudging across the lawn to the castle door for the feast.

A hissing sound from behind him made Sirius whirl in his tracks. It was the cat. Sirius communicated the plan, and together they set out across the grounds. The cat slithered into a small hole near the stairs leading up to the large wooden doors and Sirius breathed a sigh of relief that he had lost so much weight in Azkaban. The hole led to a passageway just large enough for a house elf and intersected with the passage from the one-eyed witch to Honeydukes that Sirius had discovered in his second year at Hogwarts. Pausing behind the statue, Sirius could hear numerous pairs of feet crossing the hall. The whisper of a girl caught his attention just before the sound of the crowd died down.

"But if he—you know"—the girl's voiced dropped lower still—"if he was trying to—to poison Lupin—he wouldn't have done it front of Harry."

"Yeah maybe," a boy's voice responded. Sirius couldn't help but think that the voice had sounded an awful lot like James. Could it have been Harry? But...Lupin? Poisoned? Who would be trying to poison his best friend? This task might be more urgent than he had thought. Squeezing around the ginger cat in the tight space, Sirius clawed urgently at the back of the statue, but the cat hissed and jumped in front of him again, just as another pair of feet scurried past. The cat gave Sirius a look as if to say, "See what would have happened?" Finally, there was silence. Sirius shoved the old crone forward and padded softly into the corridor, the bandy-legged feline taking up post behind him.

Sticking close to the wall, Sirius and the cat made their stealthy way through the castle. Sirius made sure to walk directly beneath the portraits so as not to be seen and have word get back to Remus that there had been a large black dog in the castle. Remus would understand immediately and Sirius's chance for revenge would be lost. What he wouldn't give for James' Invisibility Cloak. He wondered vaguely as he neared the portrait hole what had become of it.

Just before he rounded the final corner to the Gryffindor common room, Sirius transformed with a faint pop. It wouldn't do to let the Fat Lady know he was an Animagus. From around the bend, a woman's voice called, "Who's there? Forget something on your way to the feast?"

"No," Sirius rasped, stepping out of the shadows and standing before a portrait of a rather large woman in a pink frock.

The Fat Lady's eyes widened in horrified recognition. "Sirius Black!" she whispered in terror.

"Yes." Sirius answered, sliding a long knife from within his robes. "I used to live in this tower, remember? I need you to let me in once more; I have a score to settle."

"I won't let you in without the password," the Fat Lady cried stubbornly and added on a slightly hysterical note, "especially when you aim to murder that poor Potter boy! Well, not on my watch!"

"Murder Harry...?" Sirius began, bewildered, but an insistent hiss from the ginger cat standing guard around the corner cut him off. He had to hurry. "Look, woman, I've no time for chit-chat, let me in the tower!" he growled, brandishing the knife menacingly.

"No!" The poor Fat Lady looked absolutely petrified, but heroically stood her ground. The next moment, Sirius attacked. In fury and desperation, he slashed at the canvas, the knife flashing dangerously. He would not be thwarted in his revenge by a stupid picture. His freedom was not the only thing at stake. If Peter ever got word that Voldemort was regaining power, Harry's life would be in grave danger.

The Fat Lady burst into terrified sobs and fled, just managing to dodge a particularly vicious slash of Sirius's knife. Her wailing could be heard as she tore through the paintings on the floor above, thinking only of escape. Finally, Sirius tired and stood hopelessly in front of the tattered canvas, still locked out. A gleeful cackle caused Sirius to whirl around in alarm, his eyes searching for the source of the noise. Behind him and just out of reach, Peeves the Poltergeist sat cross-legged in midair, grinning nastily from ear to ear.

"Peeves!" Sirius moaned, near panic. If anyone would love to see him caught, it would be Peeves.

"Can't get into the tower, old Blackie-poo?" Peeves sing-songed happily. "Should call the dementy-whatsits, I should. Oooh, they'd love to see you, yes, oh, yes they would!" A mischievous glint appeared in Peeves' beady eye.

Sirius paled visibly. "Please, Peeves, no!"

Peeves grinned wider and danced a little jig in the air, "Better run, Blackie-wacky! Feast's almost over, it is! Oh, you better run or I'll scream and then you'll be for it, yes you will!"

Sirius turned and bolted, Peeves' mocking laughter echoing behind him. As soon as the annoying little man was out of sight, Sirius transformed back into a dog and ran full out, panting desperately for air. As he neared the statue of the humpbacked witch, he could hear the footsteps of the Gryffindor students on their way back to the tower. He stopped short, nearly running headlong into the statue, and began scratching at the base with his huge paws.

He needed a wand to get it open! How could he have been so foolish! The cat streaked toward him from the direction of the Great Hall, the hair on his bottle-brush tail standing on end and a stolen wand clamped between his teeth. Sirius reached out his paw—now a hand—as the cat reached him, snatched the wand and tapped the witch's hump with it, muttering, "Dissendium." The wand fell to the floor as a bear-sized black dog slid into the passageway, the entranceway snapping shut behind him.

Sirius heard pounding footsteps, then an angry voice yelling, "Give me back my wand, you crazy cat!"

A half an hour later, Sirius collapsed on the dusty, rickety bed in the Shrieking Shack, utterly exhausted. Peter was still free, and there was no way to get into that tower without the password. He would just have to find a way to get it.