Bill Cipher's Nightmare

Everyone knows that dream demons can't stand being near or touching a dream catcher. Humans see such beauty in these objects, dream demons however regard them as deadly symbols, something to be feared. In the darkness of Bill Cipher's room he has a nightmare one that leaves him shaking. What could that nightmare be? (Horrible summary I know, but this was just an idea I came up with in the spur of the moment ^^; )