Not much is known about The Prophecy, except that it talks about the Pure Evil One and him using The Book of Pure Evil to end the world. Presumably, it is far more complex and open to interpretation. Obviously the people most concerned with it are satanists and share the same interpretation.

It is unknown where this prophecy comes from. Both Atticus Murphy Sr. (The Hooded Leader) and Pat Kolinsky, Jenny's father, knew about The Prophecy, but both of them passed away before they revealed enough relevant information. The Metal Dudes also know a thing or two about it, and even wrote a song about it, simply named "The Prophecy". Jenny incidentally found the album that contained this song in episode 2x11 and listened to it; the following fragment was heard:

For the end of the world to happen the Pure Evil One must do three things:
Number one, the Pure Evil One needs to hold The Book of Pure Evil.
Number two, the Pure Evil One needs to be filled with anger and frustration.
Number three, the Pure Evil One can't bear the mark of the loser, and loser means virgin; you can't be the Pure Evil One if you've never done the nasty.