What is the main conflict in Borrower of the Night by Elizabeth Peters?

Borrower of the Night by Elizabeth Peters is a story about Vicki Bliss, a extremely beautiful woman who is also brilliant and self reliant. Vicki and her friend Tony are reading and find clues that hint at what happened to an art piece that some believe to be a myth. Vicki and Tony head to Germany to find the piece but soon discover they are not the only ones after the relic. Things become quickly dangerous as it seems a ghost haunts the castle where Vicki and Tony are staying and conducting their research. The desire for the relic drives people to desperate measures and Vicki and Tony are nearly killed, along with others in the story, before the art piece is found and returned to its rightful owner.