I'm not sure specifically as there could be many different 'Crimson Rivers' Harry Potter fanfics. Generally, it might involve new adventures in the Harry Potter universe, perhaps with a focus on a mysterious 'Crimson Rivers' element. It could be about new characters related to it, or a new plotline that twists the original story in a way related to these crimson rivers.
One interesting story could be about the characters solving a mystery that was not fully explored in the original work. For example, they might discover a hidden society within the town that has been causing all the strange events.
One idea could be a prequel fanfic that delves into the backstories of the main characters before the events of the original. For example, exploring their early days in the police force or their first encounters with the strange cases that would later shape them.
They are likely to be horrified. James, for example, would probably be extremely angry that his friend had to endure such a thing. He might immediately vow to protect Sirius from any further harm. Remus, being more empathetic, would be there to comfort Sirius and offer his support in a more quiet and understanding way. Peter, though not as strong - willed as the others, would still be shocked and follow the lead of James and Remus in trying to make Sirius feel better.
First, re - watch or re - read the original Crimson Rivers to get a good feel for the characters, the setting, and the overall mood. Then, think of an idea that you are passionate about, like a new adventure for the characters or a different take on an existing plot.
Think about the story you want to tell. Is it a continuation of the existing plot? Or something completely new? Decide on the main plot points, like the mystery or conflict your characters will face.
You can try looking on popular fanfic websites like Fanfiction.net or Archive of Our Own (AO3). They have a vast collection of Harry Potter fanfics, and you might be able to find the 'Crimson Rivers' one by using the search function on those sites.
The marauders' reaction would be quite complex. They might feel a sense of nostalgia as they read about the world they once knew, but through a different lens in the fanfiction. They could also be a bit critical. If the fanfiction didn't seem to respect the essence of the original story, they might be quick to point it out. However, if it was well - written and added new depth to the story, they could end up really enjoying it and even discussing it at length among themselves.
Remus might be a bit more reserved. He'd approach the fanfics with caution, but he'd also be touched by the stories that explored his struggle with being a werewolf in more detail. He might find comfort in knowing that fans understood his inner turmoil to some extent.