What do you want to do with it? I'm planning on doing a "phantom haunts the target, only visible to them." I want to pair that with reports that there's a magical curse that causes hallucinations, eventually leading to violent insanity (with magical abilities), which the only cure is to kill the infected.
Then I'm going to have the hallucinations become more and more extreme, make the player freaked out, and paranoid that the rest of the party figures out.