You see a monster big bang pop up in tags and it's easy to get confused with all the event names. Basically, it's a collaborative fan event, usually community-run, where writers and artists team up. A writer produces a long fic, think novella-length, and an artist creates a piece based on that story. They post together, a big showcase.
The 'monster' part just means the word count target is huge, like 20k, 30k, or even more. It's a marathon. Regular big bangs might have lower thresholds, say 10k. The vibe is more epic, both in commitment and final product. I helped beta for one set in the 'Star Trek' AOS universe once, and the writer was pulling all-nighters to hit 25k.
What sets it apart isn't just length, though. The coordination feels different. With that much story, the artist has tons to work with, so the art often feels deeply integrated, not just a cover. It's less about a quick gift exchange and more like building a small monument together. You get these incredibly detailed fics that might not exist otherwise because the event provides the external push. The downside is you see fewer of them, for obvious reasons. Burnout is real, but when they click, they're stunning.