Greek mythology

The Greek myth of Prince Theseus and the trials that befall him when he vows to become a hero.

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. 

After fourteen-year-old classics prodigy Jack Perdu has a near fatal accident he meets Euri, a young ghost who introduces him to New York's Underworld, where those who died in New York reside until they are ready to move on, and Jack vows to find his dead mother there.