The question is unanswerable because it mangles Shakespeare into something ungrammatical.
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He says in comes out of the ocean at night. :P
After the boy's had mocked Percival's report of seeing the beast, by telling him no large animal could live on such a small island, Percival gave them pause for thought when he told them that the beast came from the sea.
It is called ectoplasm, and it comes out of the body when there are powerful séances.
Simon comes to tell the boys that the "beast" is actually a dead parachutist on the cliff and that they don't have to be afraid anymore. But the other boys are caught up in the reenactment of the hunt of the beast, acting out the drama of what they would do. They all get lost with a sort of mob mentality and do something that none of them would normally do. They end up savagely murdering Simon, viewing him as the "beast".
That quote comes from the internalised conversation that Simon has with the beast within himself, during an epileptic fit. It is from chapter 8: Gift For The Darkness, a couple of paragraphs from the end, on page 158 in my edition of the book.