what does simon discover about the beast and what is his reaction?

At the beginning of Chapter 9, Simon has just come out of the faint he experienced when the "beast" spoke to him through the impaled head of the boar. He's slightly dazed and senses that a storm is rolling in so he moves towards the mountain like a man in shock. Crawling up the side of the hill, Simon sees what he thinks is an apparition, but then suddenly realizes that what lay before him is the dead pilot. The sight is an epiphany.

The dead body changes everything; the boys have mistaken the movement of the parachute in the breeze for a deadly beast, a piece of canvas has plunged the entire group into chaos. The sight of the dead parachutist causes Simon to become sick; he vomits, and then realizes he must inform the boys at Jack's feast of his discovery immediately. Simon moves towards the distant fire to tell the boys, but all that awaits him there is a savage death.