Diana Wynne Jones 
Drowned Ammet 

The people of Holand in South Dalemark are bitterly oppressed by the tyrannical earl Hadd. Informers, secret police, and cruel rent-collectors terrorize the countryside, and Mitt has grown up with more than enough reasons for joining the freedom fighters.When his protest against the tyrannical government fails, a young boy escapes, with two other children, to the mysterious Holy Islands where they learn the identity and the power of two folk figures celebrated by their countrymen.