The beast is unintentionally described as darkness several times. Ralph has ash in his eyes and is having a hard time making anything out because they are in the dark, looking at the dark for something that is probably... dark (in more than one way). Readers understand this through these words that preface what they do find:
If there was no beast—and almost certainly there was no beast—in that case, well and good; but if there was something waiting on top of the mountain— what was the use of three of them, handicapped by the darkness and carrying only sticks?
Moments later they speak to it and in return get this:
Out of the darkness came the answer.
Next, Ralph and Jack exchange sarcastic words with each other and the beast becomes a stain, or rather Jack:
A stain in the darkness, a stain that was Jack, detached itself and began to draw away.
Here, it is as if Jack is becoming the darkness, or the Beast.
Jack, in drawing away, makes some noises that make the other boys think he might be the beast, and then the three boys are together again:
“I saw a thing bulge on the mountain.”
“You only imagined it,” said Ralph shakily, “because nothing would bulge. Not any sort of creature.”
Jack imagined a bulge. The boys drew closer to it and again, Ralph's eyes filled with ash. This is the final description of the presumed Beast:
In front of them, only three or four yards away, was a rock-like hump where no rock should be. Ralph could hear a tiny chattering noise coming from somewhere— perhaps from his own mouth.... Before them, something like a great ape was sitting asleep with its head between its knees. Then the wind roared in the forest, there was confusion in the darkness and the creature lifted its head, holding toward them the ruin of a face... presently the mountain was deserted, save for the three abandoned sticks and the thing that bowed.
This item had many descriptions, but the most important point to take away from this chapter is that nothing was exactly described. What they could be struggling with is the power of their own imaginations. The author also gives great suggestion that Jack could be an aspect of the Beast.