Go to DriveThruRPG, find the pumpkin, ghost, witch’s hat, candy or moon-and-bat symbols for free seasonal PDFs! Vs. Ghosts, the ICONS adventure Devil’s Night, a short d20 adventure called Horror at Gravehollow Hill, Pixel Dungeon Zombies, Haunted Locales, the slightly out of place Swashbuckling Adventures... and maybe more.
Trick or treat!
And there’s more on DT Fiction, Comics and Cards. (My first find on Comics is an RPG adventure!)
And no, not telling you where to look. Although I recommend the Halloween sale... ;)