Rules for using the Dash:
Use a dash to set off a short summary after a complete main clause:
At the bottom of Pandora’s box lay the final gift–hope.
We may also use a pair of dashes in place of a pair of commas to set off words, phrases, or clauses that interrupt a sentence with additional–but not essential–information:
In the great empires of antiquity–Egypt, Babylon, Assyria, Persia–splendid though they were, freedom was unknown.
Unlike parentheses (which tend to de-emphasize the information contained between them), dashes are more emphatic than commas. And dashes are particularly useful for setting off items in a series that are already separated by commas.
These three punctuation marks–semicolons, colons, and dashes–are most effective when used sparingly.