Here are a few ideas for wording. You can do the invitation in the form of a tri-fold brochure.
Some off the cuff examples:
"You are invited to join us. We have a vacancy, which you might fill...permanently!"
"Due to sudden and unforeseen circumstances, we now have a vacancy open that only you can fill."
"Room # 1313 is waiting for you." Or room # 666.
"Enjoy our 'unique' dining menu or order in from Tomb Service."
"Extended stay plans available (whether you want to leave or not!)"
Weary travelers voted the Chamberwell Hotel as a 4 Scar resort. It easily beat out Motel 666 as a popular final resting destination.
Relax in the Cauldron Spa's famous wood fired 'really hot tub' or splash in the blood pool.
As for actual party decor, take a look around various internet sites for photos of Disney's Tower of Terror. I am always amazed at the detail in the lobby, front desk and boiler room settings.
Make some paper tape "sanitized for your protection" strips for the toilet seat in the restroom. You can even change it to: "Satanized for your possession" if you like attention to detail.
Eric