The Viper's Nest (the 39 Clues, Book 7)

Shaken by a shocking loss, Amy and Dan flee to an exotic land and trace the footsteps of their most formidable ancestor yet: a military leader of mythic proportions. But just as the siblings begin to master the art of ancient warfare, they confront a dangerous enemy \- the truth. With the stakes higher than ever, Amy and Dan uncover a devastating secret about the Cahill family that changes everything.