romance/fantasy

I am looking for the author/title of this book: A girl comes to a castle to become a servant. She has an unpleasant encounter with a guard and is helped by the son of the castle lord, who issues dire warnings. She is later assigned to care for the lady of the castle, who is very ill. The lady eventually dies, and the castle lord unexpectedly asks the girl to marry him. She accepts. She gradually becomes aware of 'strange forces' at work in the castle. There seems to be a division between servants loyal to the lord and others who appear to be working against him. She sees 'beings' that look off-putting and dangerous, and she starts suspecting her husband of being connected to 'demons'. I believe the protagonist is named Joanna or Johanna. Anyone recognize this story?