Templars. It was said that the knights had in their possession various holy relics, including items believed to have belonged to Christ Himself.
The heroine searching for the Holy Grail hoping it will cure her illness, the hero drank from the Unholy Grail and has been cursed for 600yrs.
When I was looking for books in my tbr for this, I can't even describe how giddy I was when I read "Unholy Grail".
So far has started off deliciously dark and stormy atmospheric.