There are five Obscura Engravings. They are copies of the original Obscura Paintings made by Brother Obscura who hid them. Each of them contains an encrypted map of the original Paintings' secret location.
Mathias Vasiley in Prague had the five Engravings. Von Croy bought four of them from him. Vasiley wanted to see more money before he handed over the fifth. The fifth engraving contains a hidden map of the Vault of Trophies location.