This spell conjures a horrifying mental image in the mind of the recipient that only he can see. The image is utterly terrifying, and has the power to scare its target to death. If the target disbelieves the illusion, he merely takes minor damage. If he believes the image is real, he is overcome with such terror and fright that he has a heart attack and dies instantly.
Phantasmal Killer is the weakest of the instakill spells, as the illusion is not very difficult to disbelieve.