The disappearance and presumed death of the boy with the birthmark on his face opens up the plot to danger. This moment marks a thematic shift in the novel as well.
Before the fire is started on the mountain, the boys' time on the island has been rather idyllic. Despite the nightmares of the little-uns, the boys are happy. With fruit to eat and lots of free time to play, the boys enjoy themselves and even believe that they are lucky to be free of parental oversight. The island seems like a playground, full of possibilities.
"This is our island. It's a good island. Until the grownups come to fetch us we'll have fun."
When the boy dies, however, the reality of their danger occurs to each and every one of the boys. Without parental oversight, there is also no parental protection. Accidents can become deadly. The nightmares of the boy who dies become portentous.
This sense of danger is important to the plot and to the sense of suspence that characterizes much of the novel. The enmity that develops between Jack and Ralph is not the innocent animosity of a childhood game, but is a serious rivalry that may lead to another death.
The small boy who is not identified other than by a birthmark, presumably dies in the fire that the boys built. With this event, the plot takes a much darker turn. Violence, death and destruction will be common themes in the novel from this point onwards. It is the beginning of the boys' realisation that things can go horribly wrong on the island, that they are not really in charge of their situation and that they cannot ultimately control nature or their own primitive instincts.