Sirius was always a persuasive person, but even he couldn't convince Moony to see him as an innocent man.
After James and Lily were murdered by Voldemort, everyone assumed that Sirius was the Secret Keeper turned traitor. They wouldn't listen to Sirius proclaim his innocence, no matter how much he begged.
Remus, who always stood by Sirius’s side no matter what sort of shenanigans the Marauders got up to, had called him a filthy liar. His scathing tone was used to cover up his devastated expression, but Sirius could see through that.
Sirius felt betrayal, sure, but that was likely nothing compared to the betrayal that Remus felt as his world basically crumbled down around him. Sirius knew that Remus would be left destitute, and it would be surprising to the unregistered animagus if Remus would last a few years or so without anyone by his side.
Remus was never strong enough to stand alone. He needed someone by his side. He needed Sirius.
Sirius, after spending a grand total of nine days in Azkaban, decided that he needed to escape and find Remus. He was going stir-crazy, and all the screaming and laughing was making his less-than-stellar emotional state even more unstable. The man was smart enough to realize that he wasn't as badly affected by the Dementors when he was in his animagus form. He was still unable to escape, though, as he was too muscular to slip through the bars of his cell. He was too big to get out when he was a Grim, and he was lacking the strength to forcefully break down the bars. He didn't have his wand anymore, either. Instead of relying on his most trusted tool, he had to practice wandless magic. Sirius tried desperately to get his magic to work, but he just couldn't get anything to form. He couldn't even get a spark to form.
It was at the three-week point that Barty Crouch Jr. joined with Bellatrix Lestrange, her husband Rodolphus Lestrange, and Rabastan Lestrange, the younger brother of Rodolphus. They all seemed to smirk down at Sirius, knowing he was innocent and rotting alongside them.
It was a surprise to everyone when Barty’s parents came to visit him a month or so, and they very obviously switched Barty Jr. out with his mother with a simple, but effective Polyjuice Potion.
Sirius felt rage at that.
Crouch Sr., for all he preached about justice and honor, was no better than the Dark wizards he so abhorred. He helped break his Death Eater son out of Azkaban, all while pretending that he was a noble man. He left his beloved wife in Azkaban to die, something that didn't take long for the sickly woman to do.
Sirius made the firm decision to get revenge against the Ministry for what they did to him. He didn't even get a fucking trial, yet he was rotting away without a trace, forced to eat rats and drink the tainted rain water that occasionally pooled on the ground around his accursed cell. He had to keep his sanity by counting the nights of the full moon. Once he counted past ten nights of the full moon, he managed to slip through the bars as a human. He had grown thin, and his strength was a thing of the past. Regardless of that fact, he left Azkaban behind. He turned into his animagus form and swam with all his might. He made it to shore in the dead of the night, collapsing in a small cave by the shore. He spent another few moons regaining his strength, stealing food from wherever he could while also hiding away from the public. He constantly moved from small town to small town, trying to track down Remus.
Remus, as it turned out, was moving from place to place as well.
Sirius always seemed one step behind the other Marauder. He couldn't tell if Remus was just badly affected by his status as a werewolf or if he was constantly trying to run away from something else. It was on a night of a full moon that Sirius managed to finally catch up with Remus, who had no choice but to hunker down and transform into Moony.
Moony snarled, clawed, and bit at Padfoot.
Padfoot had to retreat, his body no longer capable of withstanding such a beating. He was too weak after the encounter to wake up when dawn broke, so he couldn't be there when Remus awoke.
Not that Remus would remember what happened as Moony. He never did, no matter what anyone may have said to jog his memory.
Sirius had to spend two lunar cycles recovering instead of chasing after Remus. He had to lay around and heal instead of chasing after the one friend he had left in the world.
Remus was surely suffering on the run, too.
Sirius, after waiting for another two and a half lunar cycles, chose to approach Remus differently. He regained his strength, changed his appearance with Muggle hair dye and fake glasses, and fakes an appearance as a somewhat odd muggle. He patiently tracked down Remus once more, something that became easier as the full moon approached once more.
Moony showed himself eventually, just as he always did at least once a month.
Sirius didn't approach right away, instead waiting until the hour before dawn where Moony would be tired and more subdued. He Apparated near the werewolf and climbed up high into the trees.
Moony knew he was nearby. Without the ability to properly find Sirius, however, the werewolf had to relinquish control over to Remus as dawn arrived.
Sirius waited for the man to finish shifting before leaving the treetops. He sat down next to Remus, who had mere shreds of robes left, and calmly waited for his fellow Marauder to awaken.
It didn't take long for Remus to stir. When he did, he immediately noticed the man next to him. He sat up with a jolt, only to release a pained whimper as his barely clotted wounds broke open.
“Relax, Moony,” Sirius said in a drawl, something that caused Remus to freeze.
Remus looked at the man again, and his face fell upon recognizing Sirius despite the dirty-blonde hair and fake glasses. Then, a dull sort of acceptance showed across the man’s face, and he laid back down.
Sirius frowned deeply. “I won't hurt you.”
“Like I'd believe the lies of a traitor,” Remus spat out viciously.
“I see you've been fed lies like everyone else,” Sirius sneered in reply. “I bet you believe that I was the Secret Keeper of James and Lily even though it was Peter.”
“Bullshit,” Remus snapped. “Dumbledore said–”
“Dumbledore is a fucking liar then,” Sirius growled, “He knows Peter was the Secret Keeper. He cast the damn Charm himself.”
Remus scoffed, but did nothing more.
“Look at me,” Sirius demanded.
Remus looked away in defiance.
Sirius grabbed the other man’s face and turned it to face his own. “I told you to look at me, and you will.”
Remus stared at him, rage and fear mixing together and flashing through his eyes.
“I wasn't the Secret Keeper,” Sirius said firmly.
“You were,” Remus argued.
“Do you want me to drink Veritaserum to prove it?” Sirius retorted. “I’m sure I could mame some up and take it.”
Remus remained stubbornly silent.
“I wasn't the Secret Keeper,” Sirius said once more, “That was Peter.”
“Then why kill those Muggles?” Remus asked.
“I didn't,” Sirius said. “I killed Peter, sure, but not those twelve Muggles. If I had gotten a trial, everyone would know that’s a fact.”
Remus stared hard for a long moment before saying, “You've always been a charmer, Sirius. How do I know that you're not charming me into a world of delusions?”
"You can't,” Sirius admitted. “You have to trust me like I trust you.”
“I don't trust you, though,” Remus said, “I can't do this.”
“You can.”
“You're right, I won't,” Remus replied before spitting out, “Stupify.”
Sirius was hit by the spell with such force he was sent flying. As his consciousness faded rapidly, he couldn't help but feel anger towards his former friend.
Remus forced himself to his hands and knees, clearly having seen better days, before Disapparating from the area.
Sirius fell unconscious, and when he woke up once more, he was surrounded by Dementors in a heavily guarded courtroom. He had shackles and his prisoner garb once more, and he knew Remus was to blame. As he was given the sentence of a Dementor’s Kiss, he clenched his fists tightly and said without a hint of hesitation, ”I hope Harry finds out the truth and curses you all.”
Without any other fanfare, the Dementors took away his soul.
Sirius Black was no more, and the path was made for a truly hellish future.