For the perfect nightmare dress up party send out the spookiest invitations telling every-one to come dressed like a freak.
When the guests arrive have your house totally dresses up with spiders,cob webs ghosts and more. have booby traps everywhere!
To finish it off have a spooky game to play...
Eg: make your guests put their hands into a box and pull out something (have the lights turned off in this process) then turn them on and see what they pulled out (there should be plenty of slimey stuff and goo everywhere).