what happens to the boy with the mulberry-colored birthmark?

He goes missing after the big fire. He is presumed dead from the chaos from the fire. 

The boy with the mulberry birthmark didn't die in the fire because he never did actually exist. He was indeed a figment of the boys' imagination. He was representative of satan because he was the first to induce fear into the group which, as you know, ultimately gave way to their savagery. That's why at the end of the book when the rescuing naval officer asks how many of them died, Ralph answers ¨Only two.¨ (Piggy & Simon). This is only one of the many symbolic plot elements Golding uses in this fantastic novel. 

The boy with the mulberry birthmark had died from the first fire that the boys had tried to create, that went out of control