I hosted my first adult only halloween party this past year and made a theme for the party of "superstitions". Because I didn't have many decorations the superstitions theme made use of things in the house i did have like umbrellas, stuffed owls, number 13 jerseys, ladders, black cats etc....my invite was a simple black cat shaped card on which i wrote in white gel pen:

Dear oh dear
I broke a mirror
walked under a ladder to boot
Our home is bewitched
and now I've started to twitch
cause and owl just now started to hoot

If you could make haste
and come for a taste
we promise not to serve you old fruit.

(inside) Your presence is requested in bring good fortune back to our home.
Please come in costume and comfy walking shoes(they were going on a neighborhood tour/scavenger hunt) and expect a night filled with devilishly good fun. Please rsvp by said date with a head count of your kin!

Even months after when i see people they still talk about how great the party was and inquire what i'm planning next year. It was a good party theme for a first year party thrower who didn't have all kinds of decorations and an unlimited budget. Amazing what a hanging a few open umbrellas inside with errrrie lighting can do!