The Hollow Kingdom

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.

Synopsis from Open Library.