From Origins

Faced with an impossible choice, Skylar and the other descendants embark on a deadly quest to the underworld to confront Hades, who has been plotting to kill them all.
 
They discover the ancient gods are not dead, as once believed, but are instead held captive in a secret location. In order to save the world from destruction at the hands of Hephaestus, the descendants must rescue their ancestors, the gods of Olympus.
 
Skylar learns more about her bloodline and her powers, but there is a heavy price to pay. The darkest forces known to man are about to return to reclaim their stolen kingdom.