The Hollow Kingdom: Book I -- The Hollow Kingdom Trilogy (Google eBook)
"She had never screamed before, not when she overturned the rowboat and almost drowned, not even when Lightfoot bucked her off and she felt her leg break underneath her with an agonizing crunch. But now she screamed long and loud, with all her breath."
Hallow Hill has a strange and tragic history. For thousands of years, young women have been vanishing from the estate, never to be seen again. Now Kate and Emily have come to live at Hallow Hill. Brought up in a civilized age, they have no idea of the land's dreadful heritage-until, that is, Marak decides to tell them himself.
Intelligent, pleasant, and completely pitiless, Marak is a powerful magician who claims to be a king-and he has very specific plans for the two new girls who have trespassed into his kingdom. The Hollow Kingdom is a 2004 Bank Street - Best Children's Book of the Year.