What does Simon discover at the top of the mountain?

In Lord of the Flies, Simon climbs to the top of the mountain and discovers that the supposed beast is only a dead parachutist.

He had already understood that the real danger on the island was not a physical monster but rather a destructive force inside the boys themselves, and when he reaches the mountaintop he confirms that there is no living monster there. He sees the dead parachutist caught near the signal fire, untangles the strings of the parachute, and then starts back down the mountain to tell the others the truth.

Simon's discovery is important because it reaffirms that he the only boy who faces the beast directly and understands what it really is. When he returns with this knowledge, the other boys are caught up in a violent ritual at Jack’s feas. They mistake him for the beast and kill him before he can explain what he found.