Mistress of Mellyn — Victoria Holt

House of Darkness

From the moment Martha Leigh first glimpsed the cold, brooding manse high on the fog-shrouded Cornish cliffs, she felt a chill of apprehension.

Then she met her employer, the arrogant master of Mellyn, and his spoiled, headstrong daughter, Alvean, and knew why the three governesses before her had left the eerie mansion.
Slowly, almost imperceptibly, Martha began to be aware of an atmosphere of menace. There had been whispers of past horrors, warnings of violence yet to come--and now there were strange accidents. It was madness to stay on.

But Con TreMellyn's dark charm had over-powered Martha's natural caution. Against her will she felt irresistibly drawn to the handsome, enigmatic master of Mellyn.

Now, even as she found herself being pulled deeper and deeper into a frightening web of unseen terror, she knew she could not, would not leave... .