Percy and Annabeth constantly battle darkness in Tartarus, and the Doors of Death are guarded by the dark giant, Clytius. What does darkness symbolize in the narrative?

How does the switching third-person omniscient POV affect the plot and story structure?

The House of Hades is the only novel in the Heroes of Olympus series that features the POV of each of the seven protagonists. How does this serve the character development of the characters as well as the plot?