The House at the Edge of the World

Josh Flagsmith is a brave, epileptic pirate boy who lives in the Guardian's House in the west of the Western Isles, where pirates and colonists live side by side. Born with visionary powers, Josh is chosen as the Pirate Guardian, and entrusted with the onyx Stone of Truth, which he uses to control his epilepsy.
But when a cruel colonist dictator seizes power, and throws Josh's parents in prison, Josh has to escape, and sail to the island of Amaryllis, to seek aid from the Pirate Council. Helped by his friends, Sandy and Megs, and the dictator's cunning stepdaughter, Lavinia, Josh must find a way to rescue his parents and overthrow the evil dictator.
But in order to succeed, he must first learn how to use the hidden power of his stone.