What was Simon's strange encounter with the lord of the flies?

The identities of the beast and the lord of the flies are two problems with which the boys wrestle. The fact that they consider these problems seriously is an indicator of their loosening grasp of reality. The combined elements of trauma—including the effects of the plane crash and their fear of not being rescued, their limited and poor diets, and sun exposure—are all taking their toll.
The idea that a real beast is present on the island gains steam after Samneric are confused by seeing the dead parachutist stranded in the trees and Eric interprets scratches from brambles as claw marks. They do more exploring before deciding to hunt it. Simon muses on the possible qualities of this beast, imagining it as “a human at once heroic and sick”—a description that might relate to himself or to Jack. When they do set out looking for the beast, Ralph, Jack, and Roger see the trapped parachutist, which is so fearsome in the dark that they flee rather than try to understand it.
Back at camp, the disagreements about the beast’s identity continue (chapter 8). Jack and his hunters decide to lure the beast with an offering: a pig’s head. Sickened by the killing and mutilation, Simon retreats to safety and only returns after the offering has been set up on a stick and is covered with flies. The hallucinatory scene in which he converses with the “lord” is probably the most disturbing in the book. It marks the point at which the reader realizes that the boys have passed a psychological point of no return. When the “lord” tells Simon that he is the beast and urges him to come “close,” Simon finds the “darkness” inside its mouth unbearable and loses consciousness. With this dark scene, Golding also foreshadows Simon’s death.

When Simon is alone in the jungle, he witnesses the creation of the lord of the flies as Jack places the slaughtered sow’s head on a stick, which is then wedged into some rocks.
The sight of the pig’s head disturbs him. He watches as innumerable flies cover the decaying head, and he gazes in the half-torn jaw and black, lifeless eyes. As he does so, Simon begins to hallucinate. He perceives the head communicating with him, taunting him with suggestions like the following: “Run away, said the head silently, go back to the others.”
Simon responds aloud to the pig’s remarks about the beast within all of the boys. At the end of the chapter, Simon loses consciousness after the lord of the flies threatens to kill him.
Based on the clues Golding included in this chapter, it can be inferred that Simon has a condition that causes him to have seizures, which are preceded by hallucinations and other sensory issues. Thus, one could interpret his conversation with the pig’s head as a figment of Simon’s imagination.