Percy Jackson and the Sword of Hades (Percy Jackson and the Olympians, #4.5)

The goddess Persephone has summoned Percy, Thalia, and Nico to the Underworld in order to retrieve Hades' powerful sword before it falls into the wrong hands. Flip the book over for lots more ancient Greek fun with Terry Deary's 'Groovy Greeks' - it's history with the nasty bits left in!"

Note: this edition is now out of print, but you can still find this story in The Demigod Files.