After being stunned on the streets of Croydon, Sirius woke up as saltwater hit his face. He regained his senses slowly as the smell of the salt air and the taste of the water continued to splash over him. When his eyes were open, he found himself chained to a bench on a boat, surrounded by other people in similar states. Many of them were injured in some way, though they had been patched up and bandaged. Looking around, he could only see water in every direction. While the front of the boat only contained rows of benches where he and others were chained, the back of the boat had a cabin where he could see an Auror behind a wheel, steering the boat.
It took him nearly five minutes to figure out that they were heading to Azkaban. He had assumed there would be a trial, assumed he would have been paraded in front of the press as he would have been declared one of the first arrests after the fall of Voldemort. This made no sense, he had made the plan to admit to Peter's crimes in court and everything before being shipped off to Azkaban.
Once he understood he wasn't going to get a trial, he made the decision just to keep his mouth shut. He learned long ago not to look a gift-hippogriff in the mouth. He just kept his head down and smiled as he would get to enact his plan sooner than later. One thought did plague him though… who had pushed him through the bureaucracy to avoid having a trial for a member of the Ancient and Most Noble House of Black? Surely his Grandfather, Arcturus, would have at least wanted to keep that from happening. His parents were probably ecstatic with joy that he had betrayed his blood-traitor friends.
Nearly an hour later, the boat entered a thick bank of fog. Merlin knew how they were steering the ship through this soup, as Sirius couldn't see his knees and could barely see his nose. When the fog broke, many of the other prisoners did too. The Fortress of Ekrizdis stood above them like a monolith to evil. A few other prisoners started to scream or cry, frantically trying to break their unbreakable restraints. Sirius just smiled as this is where he wanted to be.
Sirius began to shiver as the boat slowly circled the island. He couldn't see them yet, but he knew that the chill was coming from the dementors at the center of the fortress. The Black Family library had a book that contained schematics of Azkaban prison, so he knew the general layout already. The tower reached up hundreds of feet into the sky but was hollow in the center. Down at the lowest levels was the nest where the dementors congregated and bred more dementors. They would occasionally float up to the tops of the tower or around the outside, but the pit is where they liked to stay as that was where the feast was.
He could already see flashes of memories coming to mind of the bodies of James and Lily, dead on the floor of their house, drawn out by the dementors. A very common misunderstanding regarding dementors was that they fed on happy memories. For anyone who had any shred of intelligence at all, this was absolutely ridiculous. The spell to repel a dementor was powered by joy and love. If they fed on happy memories, sending a Patronus to protect you would be like using a fat kid eating a steak to protect you from a Nundu. Happy memories only got in the way. A dementor would systematically strip a person of all the warm and joyful memories in their mind until only the juicy center remained.
Despair.
That was their mother's milk; hopelessness, anguish, despondency, gloom, and despair. The base of their tower was where the inmates that would be there for life resided, that was the place where they feasted. Those higher up would eventually leave the prison, so they would be a light snack now and then, an appetizer, if you will, before the main course.
Knowing all this was how one could resist the drain. Hope was something you could not have in Azkaban prison. You needed another emotion to combat the dementors. The innate magic of love wasn't powerful enough without a wand to create a Patronus charm, so it was best to just feed the dementors with something strong enough that they couldn't take from you.
Vengeance.
All of Sirius's darkest memories were filled with a desire to enact pain and suffering on those that had caused them. In his childhood, which was filled with pain, rejection, and occasional torture, he sought vengeance on his parents. On the ones who killed his friends and allies in the war, he sought vengeance on those who had cast the spells. He even knew a few of the faces behind the masks that had perpetrated those crimes and hoped they would show up as his neighbors. Finally, of the despair that he felt for his best friend James, and his wife Lily, he sought an unattainable vengeance on Peter Pettigrew.
That final mission was unreachable and would likely drive him mad, but it would keep him from falling victim to the ever-present drain of the dementors. They could try to feed on his despair for as long as they wanted, but they would only get spoiled milk, spoiled by his desire for revenge. Maybe they would eventually give up on him, he had all the time he needed to figure it out.
The boat circled to the backside of the island, where there was a dock and a building with a mechanical elevator running up the side. Charms and enchantments tended to degrade around Azkaban, so muggle devices were used to replace them. The boat they were on now was powered by a muggle engine. A few decades ago, they made the prisoners paddle across the North Sea to arrive at the island.
After disembarking from the boat, the prisoners were taken to the intake office where they were all stripped bare and given striped pajamas for their stay in the prison. Then they were taken to get their photos taken.
The Auror in the picture booth was cheery, which made everyone else even more grumpy. She would say, "Hey! Let's see a big smile!" or "I don't want to see any gloomy faces, this is how your family will remember you!" It was quite annoying.
When it came to Sirius, she glared at him, and said, "I heard you were laughing up quite a storm when you were taken in. Where's all of that jolly chuckling now?"
Sirius just glared at her, while focusing on his plans, then he started to laugh, he looked at all the faces of the other inmates around him as he focused on his darkest desires and laughed even more. Even the cheery woman looked scared as she took his photograph for the records and sent him on his way.
Sirius had heard of a few people who had committed atrocities like the ones he thought they had pinned on him. He had only killed one wizard, supposedly, and a half-blood at that. In the eyes of the courts, muggles were treated like family pets. People got mad if you killed them, but only if you seemed to enjoy it or spent time torturing them would you be labeled a killer. The ones in Croydon were only collateral damage to Peter's death. Along with the betrayal of the Potters, he expected to be placed somewhere on the lower end of medium security.
So he was surprised when he was placed in the lowest level of maximum security. The floor and walls of the cell were ice cold, and he could feel his mind being pulled at by the aura of the dementors even before he was placed in his cell. As soon as he was alone, he immediately transformed into his animagus form of a black Irish wolfhound.
While animals would react instinctively to the aura of a dementor, their minds were not the same as a human. Even though they were less complicated, the dementors seemed to ignore them as less desirable to feast on. A less complicated lock can't be opened by a modern key and the dementors were always on the hunt for human beings and accepting nothing less.
Sirius sniffed the air and took in how many people were around him in the wing. He knew the scent of a few of the Death Eaters present, as he had helped take them down. None of the important ones were there yet. However, if Voldemort was truly dead, they would be here soon enough.
As the days went by, many of them came in, fresh from their trials, crying for mercy from the terror of the dementors. News reached him, even in the lowest levels of those who had been admitted. Not just the marked Death Eaters; the ones inside or close to the inner circle but those who were unmarked as well. There were snatchers, those aiding and abetting, and even a few who thought they were working for the ministry but were just used by Voldemort's forces.
It seemed that all of the inmates who ended up on his floor were there for casting the Unforgivable Curses. It was a bit strange that he was the only one there who hadn't, though it would give him easy access to them down the line. The ones that were with him, he would wait until they were broken before ending their lives, he wanted to wait for all the trials to end starting on his mission. Sirius wanted to start with those who weren't there for life, the ones who might be there for just a year or two, for 'small' crimes against muggles and aiding Voldemort through other means.
The food was disgusting, but it was always delivered by a guard escorted by two Aurors casting their Patronus charms. He could always feel the warmth of them coming, so he was never caught in his canine form. At first, he ate the disgusting food for the two meals a day it was served, but he soon switched to one meal a day as he wasn't losing enough weight to slip through the bars in his dog form.
Nearly a year passed before the last group of inmates arrived that he had been hoping for. His cousin Bellatrix, her husband, and his brother entered the prison, like conquering heroes. Trailing along after them was a surprise - Barty Crouch Jr. Sirius had heard that the boy's father had been one of the Aurors who had taken him in after stunning him on the street, so to see that the boy was one of Voldemort's goons made him laugh. That laughter drew the eyes of Bellatrix as she marched by his cell, giving him an indecipherable look of confusion, like seeing him for the first time.
She didn't say anything to him, just moved on after a guard shoved her towards her cell down the hall. When it reached his ears what they had finally been arrested for, his thoughts of revenge boiled freshly to the surface. The four of them had tortured two of his friends, Frank and Alice Longbottom until they went insane. Briefly, Sirius's thoughts turned to Harry. After him, Harry was supposed to go to the Longbottoms to be raised. He felt a small pull on his thoughts of Harry as the dementors felt some bits of hopelessness from the thought of Harry without someone to take care of him, but Sirius quickly spoiled their meal with plans to murder the four new residents of the prison.
With the full party assembled, he could put his plan into action. Sirius was now skinny enough to slip between the bars in his animagus form. At night, when the human guards dared not to be anywhere but in the guard room at the top of the prison, Sirius began to stalk the halls, going up only to the top of the medium-security floors before returning to his cell. The ones he knew of or had heard of that helped on the side of Voldemort, he would stop at their cells and growl until they spotted him, then moved on.
During the daytime, he heard stories passed around that death, in the form of a Grim, was stalking the halls of the prison, selecting those that would be killed. The guards weren't taking anyone seriously and laughing at the stories like a joke. He even heard one guard say that he'd love to see a cute doggy after being trapped with dementors for so long.
Finally, when the legend had been spread enough, he made his first move. He had picked his first target carefully. Garret Snakebark, in medium security, the man was in Azkaban for two years for the rape and murder of a muggle woman as part of a Death Eater initiation. Sadly for him, he had been caught before he could return to his master to be marked. The man had been in the prison for almost his entire sentence and Sirius estimated that he would be released within the next month or two. Release was not in this man's cards, he would be the first to fall victim to The Grim of Azkaban.
Sirius had left him alone for nearly a week after visiting him as The Grim, but tonight, he slipped out of his cell and stalked quietly, like a wraith through the halls and stairs until he arrived at Garret's cell. He must have made some friends with the guards as his cell had a few luxuries added to it. He had a picture of his family on the wall as well as an extra blanket and a calendar. Days were checked off through the month showing that he had arrived just in time as the man was going to be released in three days.
The man was sleeping peacefully... too peacefully. Anyone with blood on his hands, like him, should have nightmares while sleeping in Azkaban, at the very least. Sirius entered the man's cell and crept silently to his bedside. As quietly as he could, he opened up his jaws and made sure his teeth were completely around the man's neck before biting down hard. The murderer barely had time to wake up before his throat was completely crushed and his neck broken.
When the guards discovered his body in the morning while delivering breakfast, they found a skull painted in the man's blood on the wall. A skull with the print of a dog on its forehead.
Later on, someone also noticed a bloody X on his calendar to mark the day of his death.
