The pig's head, in William Golding's Lord of the Flies, exists as the most important symbol of the text's entirety. The pig's head is "The Lord of the Flies." The fear associated with the pig's head, especially the fear felt by Simon, illustrates the tone and mood established by its inclusion.
The pig's head symbolizes both literal (a pig's head on a stick) and figurative (evil power) concepts. The pig's head evokes such fear and represents such a deep evil that Ralph finds it necessary to take the head of the stick (since he finds its bony grin unnerving). In the end, the pig's head is not completely "unthroned." Instead, Ralph only succeeds at knocking it off the stick. Unfortunately, the grin which unnerved Ralph is even bigger now.
Since the pig's head represents evil, the tone and mood reflect this by inciting fear in the boys. Their fear translates into fear for some readers.