Oh man, there are actually a ton of exchanges these days. Though this is the only large multifandom one of I know that's halloween-themed.

The signup + posting part is on Ao3 (here's the collection, but the announcements/organization stuff is done on dreamwidth over at trickortreatex. As soon as Halloween rolls around, there'll be a ton of new fic...! I'm hoping there'll be a lot of stuff for horror canons that I'm familiar with, but it's hard to predict.