The weirdest book series I've ever read would have to be

the Order of the Sanguines series, where an archeologist and a soldier are forced to team up with vampire priests in order to find a book Jesus wrote with his own blood and give it to Satan/Lucifer/the Devil himself. Yes, you read that right.

Elizabeth Bathory and Rasputin make appearances, too, though she's one of those characters who could be completely removed with little to no change to the plot. I've read the series two or three times now and I'm still not really sure why she was included, other than her being the "blood countess".