Where does Percival say the beast comes from?
Percival Weyms Madison is quite an important character in the novel, as he, perhaps more than anyone else, is a clear yardstick for how far the boys have descended into savagery. It takes a while at the assembly to actually make him speak in public, but eventually he does, revealing what he knows about the beast:
Ralph leaned forward.
“What does he say?”
Jack listened to Percival’s answer and then let go of him. Percival, released, surrounded by the comfortable presence of humans, fell in the long grass and went to sleep.
Jack cleared his throat, then reported casually.
“He says the beast comes out of the sea.”
Percival thinks the beast comes from the sea. And, with that thought, Golding cleverly shows us each of the boys becoming slightly frightened of the vast darkness of the sea which surrounds the island:
The last laugh died away. Ralph turned involuntarily, a black, humped figure against the lagoon. The assembly looked with him, considered the vast stretches of water, the high sea beyond, unknown indigo of infinite possibility, heard silently the sough and whisper from the reef.
And then, just at that moment, Maurice cites an adult's opinion which makes the sea seem even scarier:
Maurice spoke, so loudly that they jumped.
“Daddy said they haven’t found all the animals in the sea yet.”
Step by step, you can feel the fear spreading. Of course, there's nothing there.