Any religious imagery should work well with this theme. I'm reminded of the statue of Virgin Mary that was defaced in The Exorcist. You probably don't want to go that extremely sacrilegious, but slightly altered images are effective. The zombie Jesus pendant is a perfect example.
A general exorcism theme will give you more freedom, but if you want to draw some ideas specifically from the classic movie, The Exorcist, a few things come to mind.
Of course there's this classic image of the priest arriving at the house, bathed in light. You invitation seems designed to mimic this, so nice job there.
To take this silhouette idea further, you could hide a light behind something (your bar, a box, a pumpkin) and aim the light toward a wall, with a cut-out shape in front of it. Cast a cross-shaped shadow on the wall?
Of course, this can also be done by cutting out the shape on the back of the pumpkin, and placing it near a wall so the light will form the shape behind the pumpkin.
Or if you wanted to go even deeper with the mythology of the movie, there's always the statue of Pazuzu, one of the demons that possessed Regan in the film. Even appeared in her room, again silhouetted.
Speaking of Regan's demons...
I ran across this wall sculpture in my web travels. I can't find anywhere to purchase it, but wouldn't it also be perfect for your party theme? I'd love to have this on my wall.
And speaking of silhouettes... as Queen might say....
"I see a little silhouetto of a man"