What is the main conflict in Siege of Darkness by R.A. Salvatore?

The Siege of Darkness is a mystical novel of warfare and victory with a cast of characters which includes humans, drow, dwarfs and gnomes. Drizzt Do'Urden, a renegade drow who has just escaped captivity in the city of Menzoberranzan, unites with his friends in the dwarf city of Mithral Hall to conquer an invading drow army.