Only Zuul
Usage
Whenever someone is looking possessed in a grab, or their eyes are glazed over in some way.
Origin
In the movie Ghostbusters, Sigourney Weaver's character is possessed by the demon Zuul. At one point when Bill Murray is trying to get through to her, she says "There is no Dana, only Zuul!"
Examples
From http://www.glitterrock.org/etc333.html
From http://www.glitterrock.org/trek128.html
From http://www.captions.50g.com
From http://www.glitterrock.org/etc2b244.html
Fred
A spin-off of the Zuul gag is Fred, Zuul's fill-in/understudy. Fred possesses people when Zuul is on vacation, or just doesnt want to possess someone.