The fanfiction is currently above 16,777,215 words (we don't know the word count because fanfic.net uses 24-bit integers to count the amount of words and this fic surpassed the computational limit), and is 2000+ chapters. It is written by Jamesdean5842 and is still being written to this day.
The story is about an OC named JD Knudson who moves into the Louds town and goes on adventures with them after creating a superhero team with them. According to the recap (Obviously not even close to finished recap) on Tvtropes.org, the fic is mainly comprised of loud house episode plots rewritten to include the OC and a multitude of crossovers with other franchises from any piece of media you could possibly think of. (https://tvtropes.org/pmwiki/pmwiki.php/Recap/TheLoudHouseRevamped)
Some examples of the crossovers are Ed, Edd n Eddy, Naruto, Dante's Inferno, Inside Out, Matilda, Pulp Fiction, CatDog, Metroid, The Simpsons. This is a very small selection of the crossovers done in the fic.
Despite the fic's main characters being minors, the story contains lots of swearing, sex, romance, drugs, and violence. The sheer amount of strange details about this fic is insane, reading the TVtropes page will give you some insane mental whiplash. (https://tvtropes.org/pmwiki/pmwiki.php/Fanfic/TheLoudHouseRevamped)
This is just the tip of the iceberg for this story, it is absolutely insane.
One of my favorite things about fanfiction is the wild crossovers. This particular fic doesn't sound like my thing, but just, people go so far off the rails in a way that is really enjoyable to me. Sometimes for "hilarious chaos" reasons, but other times people somehow manage to pull off a serious and moving story out of a blatantly ridiculous premise, which is just about my favorite thing ever.
Unrelated... Why does anyone choose fanfiction.net or wattpad over ao3 for hosting fic these days? There's definitely a cultural gap between the sites and the userbases, but even accounting for that, ao3's tag system is so much better and easier to search/better for discoverability, and there aren't ads. Are fanfiction.net and wattpad allowing authors to monetize fics or something? Or is there some other advantage I'm missing? Or is it just habit and sticking with the site they got used to when they got into fanfic?
I think fanfiction.net technically banned sexually explicit fics ages ago (idk about wattpad), so couldn't explicit fics get unexpectedly taken down there, too? Or did they walk that one back?
Edit: thinking about that number a bit more, although that may be the longest single fic, there are definitely a lot of authors who have written just as many words or more if you count all their stories within a single fandom. Some of them will write episode re-writes, similar to this, but post each re-write as a separate fic, too. And there are also a lot of long fics that are split up into series even though their plots form one continuous throughline, and I wonder of those may have been overlooked by whoever determined that this Loud House fic is longest. It absolutely might be longest, but I wonder.