What was the tragic result of the earlier fire that had left ashes on the mountain in chapter 7 of Lord of the Flies?

The tragic result of the fire was that one of the littluns was killed.

In Chapter 7, the boys are frightened when they explore the area of the island that is still ashes from the fire.  The fire was started when the signal fire got out of control.  One of the littluns was never seen again after this fire.

Three were playing here now. Henry was the biggest of them. He was also a distant relative of that other boy whose mulberry-marked face had not been seen since the evening of the great fire; but he was not old enough to understand this … (Ch. 4) 

After the fire, the boys do seem to show some guilt or at least awareness about the boy’s death.  However, as time goes on they seem to pretty much forget about him and their actions get more and more violent.  Jack desperately wants a pig, and suggests using a littlun in their “kill the pig” game. 

When Jack climbs the mountain near where the fire was, he is frightened by what he describes as a “bulge.”  The others do not think anything could survive the fire, but Jack is convinced that he saw something.  The ashes make this part of the island even creepier. 

“I saw a thing on top.”

They heard him blunder against the trunk which rocked violently. He lay silent for a moment, then muttered.

“Keep a good lookout. It may be following.”

A shower of ash pattered round them. Jack sat up.

“I saw a thing bulge on the mountain.”

“You only imagined it,” said Ralph shakily, “because nothing would bulge. Not any sort of creature.” (Ch. 7) 

The boys’ fears about there being a beastie on the mountain seem to be materializing.  The choices they have made have had consequences.  The boys have already caused the death of one child, and there will be more to come.