Fantasy book about a vampire who sacrificed his family for power

Hi, I can't remember the title of the book but I remember some stuff from the plot so maybe someone can help me find it.

The story was about a hero who was supposed to kill some sort of dark lord. Our hero is in love with priestess/witch who's later put in danger. They join forces with dark lord and embark on an adventure to rid off actual source of evil. I remember that dark lord sacrificed his family for magical power. The book started with POV from his wife coming from horse riding trip to the empty palace and finding their children dead.