For the manga and anime fans that use the library (and particularly if you have an established manga group) run a Halloween cosplay.
There are a number of gothicy, scary manga series including Rozen Maiden, Rosario Vampire, Hellsing, Reiko the Zombie Shop, Kurosagi Corpse Delivery Service, Cat-Eyed Boy, Hell Girl, Zombie Loan.
You can also encourage your manga fans to dress up as Sadako Yamamura the ghostly girl from The Ring movie. “within seven days of watching a normal videotape, you receive a phone call, saying you will die in a horrible and painful way” you could play pass the parcel with the “prize” being a video tape cassette.
Encourage the artistic members of the group to design and create their own manga horror characters. Almost anything that you can do during a standard manga meeting can be adapted for a Halloween special…