In Chapter 9 of "Lord of the Flies" by William Golding, Simon personifies an impaled sow's head surrounded by flies as the "Lord of the Flies" of the novel's title, which he equates with the evil that lives in the hearts of all mankind. The sow's head was impaled in this way by Jack and his followers to placate a personified threat they describe as "the beast." This beast is in fact the rotting corpse of a soldier.
It is the personification of the dead soldier as a monster that galvanizes Jack's followers behind him. They view it as both a threat and a god to be appeased, and it becomes a tool of fear to be wielded by Jack to maintain control of the boys. Even those outside of Jack's group are fearful of the perceived monster, with both Ralph and Piggy, although somewhat more skeptical, feeling disturbed by it.
The scene in which Simon faces the Lord of the Flies has been compared to Jesus' encounter with Satan in the Bible. The impaled sow's head is interpreted as a personification of the evil that has emerged among the stranded boys and, by extension, the evil that naturally emerges within any society.