Author's Note:
This is actually super old. I think I wrote this either in 2012 or 2013, but I found it in my google drive documents from back in the day when I played Sirius on a roleplaying site. I kinda liked it, and thought I would immortalize it here. ;)

May 1st, 1982

Six months had passed on this day exactly since he had been imprisoned for life in Azkaban. He knew because he had been keeping tallies by scraping the edges of his fingernails on the side of the battered stony wall of his cellar. Unlike the old man who was also behind bars in the cell across from him - of whom he chose to speak with only when they made their usual bet on the time of day - he didn't want to lose track of stolen time like so many other prisoners around him had. Even if it meant breaking the underside of his nail beds routinely, cracking the skin open until his fingers bled, and painfully cleaning them up with a small cloth they offered him to keep himself the barest of unsoiled in appearance... Time was something that was very important to Sirius Black, and he wasn't about to lose track of it.

After all, he planned to count every day, hour, minute, and second until he could finally attain his vengeance against that traitor Peter Pettigrew.

The cellars in Azkaban were divided by an upfront passageway, although Sirius couldn't tell for certain how many wizards and witches there were around him. Each personage was separated only by the bars and the thick boulder-like walls. It didn't seem like much, especially not for a wizard capable of magic, but with how much enchantment and protection there was to ensure that no one could escape, there wasn't a wonder in Sirius' mind why anyone hadn't succeeded to break out. The men who were on each side of him had both been sucked dry of their souls, and the aforementioned old man tended to ignore him if he asked any questions. Since he had arrived, he had only been given two sets of clothes that were cleaned once a week. They were fed three times a day regularly. Although, Sirius wasn't certain if it was salt and peppered cat vomit that they were serving him. He tended to only eat what he was given when he was on the brink of starvation and able to properly stomach the food without gagging. Sometimes, on a good day, a rodent of sort would wander into his prison cell, in which Sirius would quickly transform himself into his Animagus form and swallow the mouse whole before becoming a human again. The thought of eating one of them had been repulsive at first, but now the tiny fury creatures were something of a delicacy.

The prison floors were covered with unbelievable filth. Sirius swore that it was the dirt and grime that covered his body that kept him even slightly warm during the cold winter evenings when he attempted to sleep. Far more than the thin sheet they had offered him as a blanket, anyway. There were several nights where the cold was so unbearable that he forced himself to sleep underneath the shriveled mattress to try to retain body warmth. The Dementors didn't help the situation, either. Four times a day, the hooded black figures glided through the dark passageway and made their rounds to stare deeply at the prisoners between the bars. Ever since he had been a boy, Sirius hadn't been frightened of many things, but he could honestly say after his incarceration in Azkaban that Dementors chilled him to the bone. As soon as he overheard their robes - dragging cryptically against the floor of the prison - Sirius made a chronic habit to throw himself on the top of his bed, squeeze his eyes shut, and wait for them to pass. Even if he felt drained of whatever little happiness he had left, he couldn't bear to look at them anymore.

Azkaban had changed him already, even if only 181 days had passed since he had last been a free man. For the first month, Sirius had been optimistic in hope that they would eventually pull him out for a trial. He begged and pleaded to anything that did so much as brushed the bars of his cellar, imploring with anguish that they would see reason and understand that damn Peter Pettigrew was alive...! But there wasn't a soul that believed him. Finally, after one full day of consistent trashing in his own prison cell, the withered old wizard from across the passageway finally stood up to shout him down for the final time.

"Y'don't geddit, d'you, boy?! They ain't lettin' you out of 'ere, understand?! Yer gonna be 'ere for life, just like the rest of us! So sit down and quit yer whining - yer not goin' no where!"

the first time in his life, Sirius had silenced himself when another individual had told him to. And that was the last moment in Azkaban where he had tried to pray his way out. Because it was clear from that moment that the man was right...

The Ministry was never going to give him a fair trial, nor allow him to defend his actions and prove that he was an innocent man. And while this realization should have made him lose any will power he had left, it was instead the awareness of his best friend's death that had slowly eased its way into his consciousness shortly afterwards. The loss of James and Lily didn't feel real until he knew that neither of them were going to show up and defend him.

They weren't going to pop up any time soon, nor any time in a late future, either.

James wasn't going to appear, laughing in front of his cellar and say, "Oi, Pads. Learned your lesson this time, eh...? Joke's on you! We're not really dead, mate! I can't believe you would be gullible enough to believe all of those silly rumors!" And Lily wasn't going to Apparate behind him and chastise her husband with, "James...! That really wasn't funny whatsoever! It's about time you boys grew up a little - what kind of examples do you think you are to Harry?!"

Them. All of that. It wasn't there anymore.

After three months, the repetitious dreams of his friends' miraculous survival began to fade. He began to stop dreaming whatsoever, which was admittedly another thing that scared him. What if he forgot what they looked like...? James, with his messy jet-black hair and slightly lopsided spectacles... Lily, with her brilliant red hair and vibrant green eyes... Those faces were the last bit of hope that he had to cling to, and the more he thought about not seeing them ever again, or being in the Potter household... Sirius began to sob fat tears for the first time in years. Since the war had begun, he had lost many friends, and even his own brother. Death wasn't a rare occurrence when it came to the Dark Lord, but he had never imagined that James and Lily would have been the victims of the war. They were good people, if not the best that Sirius had ever known, and he felt guilty. He had been the one to suggest Peter as secret-keeper. He had been the one to shun Remus instead of suggesting the lycanthrope for the job, who - Sirius realized wholeheartedly now - would have been a far better candidate than that disgusting traitor.

The overwhelming despair and remorse inside of him quickly curved into what was anger. Sirius spent hours of the day beating his own knuckles into the hard walls of the cellar. Eventually, just like everyone else around him had, he stopped keeping track of the time on the wall. Instead, his gray eyes - which had become accustomed to deliberate darkness - paid more attention to the dried pits of blood that had gashed over his knuckles. The purple bruises that covered his hands, and the way he noted how thin he had become over the period of time... His hands felt around his cheeks, realizing how much weight he had lost by how sunken in his cheekbones felt... The boy he had been in Hogwarts felt so long ago, and he was beginning to forget what he had looked like in that period of time. Even so, he didn't feel like a man, because he knew if he was one, he would have accepted that James and Lily were dead by now...

And that he should be grateful that his godson had survived, even if his best friends had not.

A strange, exhilarating feeling of hopefulness began to bore into his senses when his dreams finally returned. And it was one of that little boy with the tousled black hair who had so strongly resembled his best mate had remarkably managed to defeat the dark Lord Voldemort that had shaken Sirius back into faith. He had dreamed that the baby with the scar had grown up into a young man. And he was safe, he looked healthy, clean, and he was the best wizard of his young age. And for the first time, with the memoir of that baby who was the last living memory of his best friend, Sirius had woken up with the smallest of all smiles on his cracked lips. To his delight, on that morning precisely, he had been allowed the newest edition of The Daily Prophet for good behavior.

As soon as his eyes had adjusted and began scrambling over the words and reading whatever he could as quickly as he could, the date of the newspaper caught the corner of his eye. Blinking, Sirius' lips contorted into that of its old mischievous grin when he read what day it was.

July 31st, 1982

Very quietly - his voice gravelly and deep, rough from misuse and abandonment - Sirius whispered, "Happy birthday, Harry." No one heard him but himself and his swelling pride. And from that day on, he swore that he would get out of Azkaban and seek revenge on Peter Pettigrew. Even if it was the last thing he ever did.