How was the "prophecy" given by The Lord of the Flies fulfilled in Chapter 9?
Chapter nine is a pivotal chapter in this novel, and it's no coincidence that it follows a chapter in which one of the main characters (Simon) "meets" the Lord of the Flies.
The prophecy is actually more of a threat--if Simon doesn't leave things alone, the rest of the boys will kill him. The language is a bit more veiled (he tells Simon he's "not wanted," and he uses the words "do you" instead of mentioning anything more graphic and murderous), but the meaning is clear. And it's important to note that he names all the boys from whom we might expect such behavior--Jack, Roger, Maurice, Robert, Bill--and then adds Piggy and Ralph, who are not in Jack's tribe but are actually aligned with Simon.
Simon does not heed that warning; instead he heads for the clearing where the boys have gathered on a stormy night and finds himself in the middle of a hunting circle. He wants only to tell them what he has learned--that the beast on this island is them--but the boys are in a hunting frenzy and kill the prey which has crawled into their circle.
Simon dies, as predicted, because he was compelled to share this new-found knowledge; and he is killed, as predicted, by the boys as named. The prophecy has been fulfilled.