How does the dwarf trick Calum in "The Truth is a Cave in the Black Mountains"? Why?

When the dwarf emerges from the cave, he has no gold. In fact, he never sought gold to begin with. This was the story he told Calum to get him to take him into the wilderness where he could exact the revenge he desired against the man who left his daughter to die, tangled in a thorn tree by her hair. This is the most important of three secrets and one he has told no man.