The explanation below of how the illustration works contains spoilers. If you haven't read the book yet, don't scroll down. Come back when you are finished reading it.
- The illustration is an optical illusion. First, you see a goblin just inside a cave. There are trees overhead.
- Then you see that the tree leaves overhead are the outer edges of a giant black bramble in the middle.
- The bramble appears to be just a giant bush, but then you look at it more closely and realize that it has evil eyes, just like the goblin below.