Book 4 in the Virtual Reality series. From the back of the book: "Learning of the disappearance of your twin brother in the trackless desert, you resolve to set out and discover his fate. Leaving the hub of your elaborate Mayan civilization far behind, your quest takes you to lost jungle cities, across mountains and seas, and even to the baleful depths of the underworld. If you are lucky enough to survive the treacherous desert journey, you must still confront the wizard call Necklace of Skulls in a deadly contest whose stakes are nothing less that your own soul."