Nadia was a powerful witch known to Rowena who was a member of the Grand Coven. She was known for her ability to decode old manuscripts such as the Book of the Damned which was written in a Sumerian dialect and coded. At some point before May 16th, 1956, Nadia was killed by the Men of Letters and her codex taken. Knowing that the Grand Coven would want the codex back, Cuthbert Sinclair built the Werther Box to protect it, a box that's security system caused anyone trying to open it and anyone nearby to kill themselves. These actions got Sinclair kicked out of the Men of Letters, but he refused to divulge how to open the box and Nadia's codex was never recovered.
In 2015, wanting to decode the Book of the Damned so that he could free his brother from the Mark of Cain, Sam Winchester teamed up with Rowena who told him of Nadia's codex and sent him to find it. Using the Men of Letters' files, Sam managed to locate the Werther Box and attempted to break the warding spell. With help from Dean and a hallucination of Rowena, Sam ultimately succeeded and recovered the codex which he turned over to Rowena to decode the Book of the Damned. They later discovered that Nadia, who Rowena described as brilliant but "a selfish pig" had coded her codex so that only she could understand it, but Charlie Bradbury eventually managed to crack the code.