Nadia's Codex was a codex that could decipher, amongst other things, the Book of the Damned.
At some point Grand Coven witch Nadia came up with formulas for decoding coded old manuscripts such as the Book of the Damned and wrote them down in her codex. She was eventually killed by the Men of Letters and her codex taken. In order to protect it from Grand Coven witches looking to reclaim the codex, Cuthbert Sinclair built the Werther Box to protect it. The box had a warding spell on it that caused anyone trying to open the box and anyone nearby to suffer hallucinations and kill themselves unless they properly disarmed the spell. Despite the deaths caused by the box, Cuthbert defended his actions as necessary to prevent the codex from falling into the wrong hands and refused to explain how to disarm the box even though it got him kicked out of the Men of Letters.
In 2015, when Sam Winchester went to Rowena looking for a way to decode the Book of the Damned to remove the Mark of Cain from his brother, she pointed him in the direction of the codex as only it or Nadia could decode the Book. Using the Men of Letters' files, Sam determined the location of the Werther Box and worked to try to disarm its warding spell and get at the codex. The spell affected Sam, Dean and Suzie, causing Suzie to kill herself and Sam and Dean to nearly kill themselves, but they ultimately disarmed the spell and were able to get the codex from the box. Sam took the codex to Rowena to decode the Book and find the cure for the Mark, but chained Rowena up so that she would only use the codex to find the cure before he would destroy the Book to prevent her from using it for anything else. Rowena eventually discovers that the codex is coded too so that only Nadia would be able to read the manuscripts it was created to crack. However, shortly before her death, Charlie Bradbury managed to crack the codex and emailed her decoder to Sam so he could continue their efforts. With what Charlie sent them, Rowena was able to use the codex to crack the Book of the Damned and cast a spell to successfully remove the Mark of Cain. Rowena then fled with the Book of the Damned and the codex while using a spell to set Castiel on Crowley.
In The Bad Seed, when Sam and Dean capture Rowena again, they are able to get back the codex and Charlie's translations, but not the Book of the Damned. Sam later tells Castiel that having those should at least slow Rowena down.
In O Brother, Where Art Thou?, the Winchesters allow Rowena to use the codex again to locate a spell to safely communicate with Lucifer in Hell. However, Rowena was not granted unguarded access to the codex and it remained in the Winchesters' possession afterwards. Despite this, Rowena has since learned how to read the Book of the Damned without the codex as she continues to use the Book of the Damned without translation help.
- Season 10
- Season 11
- Sam calls the codex "a collection of supernatural code-breaking tools."
- Given what Charile said while breaking the code, Nadia's Codex is coded in saint symbolism.
- Neither the codex nor Charlie's decoder are seen or mentioned after O Brother, Where Art Thou?. Subsequently, Rowena is shown to be able to read the Book of the Damned on her own.