Much like the forbidding patch of jungle in which the book takes place (for more on that, see "Setting") the Lord of the Flies is ominous—but irresistible. Let's check out the paragraph where we hear the phrase "lord of the flies" for the first time:
Up there, for once, were clouds, great bulging towers that sprouted away over the island, grey and cream and copper-colored. The clouds were sitting on the land; they squeezed, produced moment by moment this close, tormenting heat. (8.210)
On the surface, this is a beautiful, poetic description, with "copper-colored" clouds and "bulging" towers. But if you read more carefully, these clouds don't seem so friendly after all. They're "squeeze[ing]" and "sitting on the land," not floating in the sky; and they "produced moment by moment this close tormenting heat." In other words, this vivid, detailed description produces an unmistakable sense of being stifled and oppressed, making real the tension of the moment that Simon sees the other boys kill the mother pig.
And let's check out that pig:
The pile of guts was a black blob of flies that buzzed like a saw. After a while these flies found Simon. Gorged, they alighted by his runnels of sweat and drank. They tickled under his nostrils and played leapfrog on his thighs. They were black and iridescent green and without number; and in front of Simon, the Lord of the Flies hung on his stick and grinned. (8.210)
Gross. The natural setting here is gory and ominous; the pig's severed head transforms into the Lord of the Flies. Through Golding's lush detail, we understand the boys' natural savagery. But is it totally unpleasant? Why are the clouds attractively "cream and copper-colored"? Why do the flies "[tickle] under [Simon's] nostrils and [play] leapfrog on his thighs"?