Cronus chronicles

After her cousin Zee arrives from England, thirteen-year-old Charlotte and he must set out to save humankind from denizens of the underworld, Nightmares, Death, Pain, and a really nasty guy named Phil.

As Philonecron plots to destroy the gods, transform the Underworld, destroy humanity, and remodel Olympus, Mr. Metos takes thirteen-year-old cousins Charlotte and Zee to join the Prometheans, who have an age-old weapon that may help protect them.