What is piggy's explanation of Simon's death and why does he say that lord of the flies?

Piggy explains Simon's death by attributing it to an unfortunate accident, claiming that it was dark and they couldn't see properly. He uses the phrase "Lord of the Flies" to refer to the sow's head on a stick placed in the forest as a gift to the "beast." Piggy links the phrase to the idea that the boys' fear and savagery led to Simon's tragic demise.