Board Thread:Help Desk/@comment-67.160.64.205-20130416232646/@comment-7962494-20130417000536

It is summoned only when the Guide is killed by allowing a Guide Voodoo Doll to fall into lavain the Underworld. If the Guide is currently dead when the doll falls into lava, the Wall of Flesh will not spawn because the Guide cannot die if he's already dead in the first place.

The Wall of Flesh has two eyes that shoot lasers, and one mouth in the middle. These three locations are its vital points, and they share a combined 8000 health. Damaging the eyes or mouth is the only way to kill the Wall of Flesh, attacking any other part will not do any damage. Note that the mouth has 12 defense while the eyes don't have any defense at all, so the eyes are its greatest weak point.