I can believe that people fell for the Head of Vecna in a game setting.
They have to believe a bunch of bizarre things in a game setting, so why not a head that grants you powers much like the dudes eye and hand do?
I have to admit, it was a good set up.
If you really want to freak some players out, try invisible caltrops (sp). Imagine large, invisible, extremely sharp “jack” like objects. It took them awhile to realize why they kept getting hurt when walking through a certain passage.