The Boulder of Death is a giant boulder located behind the barbecue party.
Headbutting the boulder of death will cause it to roll down the hill and into the party, although the boulder won't always hit the party. If the boulder is placed in the way of any of the vehicles, the vehicle will explode.
Causing the hillbilly to explode will unlock the "Dodge This" achievement.
A golden goat trophy is located behind the boulder.