At the end of the passage where Simon first sees the "gift" the hunters leave for the beast, what do you think is the "ancient inescapable...

In this passage, Simon is alone in his private jungle retreat when Jack and the hunters plant the sow's head on a stick right in the place where he is resting. The image of the head is extremely disturbing, so much so that even when he closes his eyes, he can still see it. Golding describes it as an "obscene thing [that] grinned and dripped." The flies are gorging on the guts of the sow,...

In this passage, Simon is alone in his private jungle retreat when Jack and the hunters plant the sow's head on a stick right in the place where he is resting. The image of the head is extremely disturbing, so much so that even when he closes his eyes, he can still see it. Golding describes it as an "obscene thing [that] grinned and dripped." The flies are gorging on the guts of the sow, then crawling on Simon. They seem innumerable. This is where Golding introduces the term "the Lord of the Flies" as the name for the sow's head.

When Simon looks back at the head, "his gaze was held by that ancient, inescapable recognition." Golding ties this phrase to the appellation he has just used for the idol-like disembodied head. "Lord of the Flies" is the translation for Beelzebub (or Baal Zebub), the chief of demons or Satan himself, according to Jewish usage at the time of Christ. An alternate meaning associated with Beelzebub is "lord of dung" or "lord of filth." Flies were considered demonic in Jewish culture; the designation Beelzebub was a designation for the devil showing how contemptible and filthy he is. Thus the "ancient inescapable recognition" means that Simon is associating the head with the most ancient understanding of evil, the very root of evil, and the epitome of everything morally repugnant and disgusting. 

In Simon's vision, Golding personifies evil and allows it to reveal itself. It confirms what Simon has understood for a while, namely, that evil comes from within humans. This knowledge is what caused Simon to suggest of the beast, "maybe it's only us." 

Interestingly, using the "Lord of the Flies" as the name of the sow's head also serves to solidify Simon as a Christ-figure and to foreshadow his death. In the New Testament, the Pharisees accused Jesus of casting out demons by Beelzebub. Jesus warned his disciples, "If the head of the house has been called Beelzebub, how much more the members of his household!" Jesus warned his disciples that they would be persecuted, flogged, and hated because of him. A similar fate awaits Simon.