Sirius came from a long line of dark wizards. Not so much evil… no that wasn't correct. They weren't overtly evil and tended to be subtle in their manipulations. Even so, they were also some of the greatest practitioners of dark magic that he had ever encountered. Voldemort may have been a Dark Lord, and while throwing out unforgivable curses like candy at a sweet sixteen party made him the most feared wizards of the age, Sirius had seen little evidence of the depths of the other evil magic he performed. However, one thing he was sure of, was that Voldemort had a fractured soul.
Sirius had been forced to study dark magic from a young age, and part of that was the theory of what murder did to the soul. While working with Dumbledore, the man had been very careful not to take lives himself in battle, and the worst that he would do is break a few bones to incapacitate someone before they were arrested. He tried to force this strategy on the other members of the Order of the Phoenix, though his efforts were often countered by Alastor Moody.
That man's strategy was to fight as if his life was on the line and if his opponent survived, then they could spend the next decade in Azkaban with a missing leg or ear. Sirius had even encountered one such victim of the paranoid old Auror in Azkaban, who was missing an arm, so he made sure to chew the other arm off after he was dead. He assumed that Moody would approve if he knew Sirius had finished the job.
Dumbledore worked his hardest to keep his soul intact, but the constant murders Sirius was performing must have made their mark on his soul. The theory said that when you killed someone, your soul cracked. Given enough time, the cracks would heal. However repeated cracks and fractures created voids in your soul, and those voids needed to get filled with something. No one knew what exactly would fill those cracks, but anything that resided in Azkaban prison, the Fortress of Ekrizdis, couldn't be good for him.
But for now, the changes in his animagus form were very good for what he was trying to accomplish. If he wanted to eliminate the inner circle of Voldemort that had been locked up here, he would need to be a lot stronger than he was. Who knew what kinds of tricks they had learned from their master that they could take advantage of without a wand.
Seeing his own red eyes in the mirror convinced him he needed to start going after some of the more connected inmates and there was one, in particular, he had been planning since the man arrived in Azkaban.
Travers.
His full name was Cotton Travers, but if Sirius had such a fluffy name like that, he would go by only his last name as well. All of the murders he had committed already were for general purposes of vengeance. Sure, he took a good deal of satisfaction in dispatching them for their involvement in the war or the deaths they had been involved in, but with Travers, he needed to do something special for him as this was personal.
Travers had been involved in the murder of a witch named Marlene McKinnon and her whole family. Marlene had been a close, personal friend of Lily Potter, and Sirius had an on-again, off-again relationship with her that had been on-again up until three months before her murder. Sirius spent a whole week sitting outside the man's cell at night, growling and chewing on the bars as Travers cowered in the corner crying at the monstrous dog trying to break into his cell. Sure, Sirius could have entered at any time, but he wanted the man to suffer first.
After a full week of psychological torture, he was staring out the window of his own cell at the night sky. Azkaban was typically completely overcast and raining all of the time. However, tonight the sky was clear and a full moon shone down on the prison.
He took that as a good omen and spoke to the heavens, "Marlene, I hope you're watching."
His transformation was smooth, almost a blending of shadows as his human form darkened and shifted into a great shaggy dog with his paws hitting the floor silently. He didn't need to squeeze between the bars anymore, his form almost melted through them at this point, not feeling the cold metal on his body at all. Then he stalked down the hall towards his target.
The last week of preparing Travers hadn't been just to satisfy his need for revenge on Marlene's behalf. He wanted the dementors to be present. The last night, one of them started to hang around, hoping Sirius would enter the cell and deliver it a snack. Tonight there were two more waiting for him outside of Travers' cell.
The trio of dementors backed off as he approached and waited as he looked into the cell to see Travers white as a ghost, huddling in the back of his cell. Instead of doing his normal routine of growling and gnawing at the bars, he just slipped right inside and growled with as much of a smile as he could do as a Grim. Sirius could see the Grim's face and red eyes reflected in Travers' face as the man broke down in tears.
The dementors were at the door immediately, the sound of them sucking at the air of the man's hopelessness and anguish like a man stranded in the desert and finding one last puddle of water. Sirius enjoyed feeding the dementors for nearly ten minutes, growling and snapping at Travers, and even drooling on the man as he curled up in a ball while begging for it all to end. Eventually, the dementors had eaten their fill and began to reach through the bars and remove their hoods to demand a soul to consume.
They would need to learn that sometimes they would not get what they wanted. This kill belonged to Sirius.
With other inmates, he had made their deaths quick as he had wanted to avoid detection, but this deep in the recesses of Azkaban, he could take his time. He stopped his growling, and sat on his haunches, then let out a small whine. Travers peeked out from between his fingers at the sitting dog, looking at him with a dumb look on its face, tongue hanging out, and was confused. He slowly sat up and stared at the Irish wolfhound in front of him.
Just as he started to regain his wits, Sirius lashed out at him, biting, snarling, slashing with the nails on his paws, filed into claws on the stone floors of Azkaban. It took nearly two minutes before Travers was lying nearly dead on the floor, bleeding out, his eyes nearly torn out, tongue ripped out, and in the corner by the window.
Before the man passed, Sirius shifted back into his human form and whispered in the man's ear, "This is for the McKinnons. This is for Marlene."
He could see the recognition in the jerk from the man hearing a human voice before he passed beyond the veil. The dementors felt his passing as well and screeched at the shaggy wizard still alive in the cell. He ignored them for now as he dipped a finger in the man's blood and painted a large skull on the wall of the room. Sirius shifted back into the Grim and barked fiercely at the wraiths trying to break into the door. The dementors stopped shrieking and backed away, as Sirius dipped a paw in the pool of blood and stood on his back legs to place a print on the forehead of the dripping skull he had just drawn.
After he was out of the cell, he growled deeply at the dementors huddled in the hallway, then marched back to his cell.
Travers' death caused the most extensive investigation so far. The man's grandfather used to be the head of the Department for Magical Law Enforcement, and he still had a good deal of pull, long after his retirement. Nearly a hundred Aurors descended on Azkaban to try and find the Grim that had been killing so many prisoners.
It was a good few weeks for the residents of the prison. With so many Aurors and Patroni roaming the halls, the cold and hopelessness the dementors brought were almost negligible. Sirius even felt the difference, though the relaxing was grating on his nerves. He didn't feel normal, spending this much time in his human form, he could feel the Grim wanting to be loose.
They couldn't find any evidence connecting him to the Grim or the killing. Just as his clothes didn't show up on his animagus form, the blood of his kills didn't show up on his wizard form when he shifted back. Another perk of so many aurors coming and going was the sheer amount of goods being traded between them and the inmates. There were certainly a lot of Aurors earning quite a bit from working the investigation and seeing the decadence in his cell, many tried to sell to Sirius, but every time he would just tell them, "Send me Humpty."
On the third day of the investigation, the man showed up at Sirius's cell and with an annoyed expression, said, "You know, everyone is calling me Humpty now. Even my own wife likes the name."
Sirius smiled back from his reading chair and replied, "But I bet she calls you that for another reason, right?"
"From what I've heard about your time at Hogwarts, that could have been your nickname too," Both men had a small laugh. "So what can I bring you today?"
Sirius frowned, "From all the deaths around the prison, I might not be here very long. Think you could get me some better food? Maybe a nice steak or some candy? Peanut butter would be nice."
"Steak would be a bit difficult, I can do some candy and peanut butter," the Auror told him.
"The peanut butter might be good if I get a visit from the Grim," Sirius said glumly. "What doggy doesn't like some of that good stuff?"
Humpty gave him a nervous chuckle, "I'll see you get it." Sirius started to pull out the blood quill that rested on his bookshelf before his supplier said, "No need to pay now, just on delivery, once I see what I can get for you."
Sirius put the black feather back down. Either the Auror was getting soft, or he was actually starting to like Sirius. He had noticed that his prices had gone down over the last few months and had been more talkative as well.
"Thanks," Sirius told him. "Watch out for the Grim, though I bet the Aurors are safe, just the rest of us here are in danger, those here for evil deeds. Those red eyes of his are terrifying… It's not natural."
Humpty gave him a nod and walked off, not knowing that Sirius was telling him the truth. Though the red eyes were terrifying… to everyone else.
