Contest dates and other bits of information of that nature haven't been announced yet, but the ArenaNet team wants you thinking of what sort of Guild Wars 2sy, Halloween-y scene you'd like to bring to life. Hit up the contest teaser page, and get those spooky and creative juices flowing!
[Update] ArenaNet has now posted the official rules for the contest! Contestants have until 3:00 p.m. EDT on Monday, October 22nd, to enter. Read the official rules for further information.