Da Vinci code. Real obscure Latinate. Gonna take me my golden years to translate it all.
Dating back to the 14th century, this journal was originally in the possession of a group of dragons. It is an instruction manual made from human skin which is encoded in an obscure form of Latinate, containing instructions on how to open a door to Purgatory intended to release Eve -- the Mother of All.
When Sam and Dean are searching for the dragons in the sewers beneath Portland, Oregon, they come across a journal in an altar, but they leave it behind when they head over to kill the dragons. When one of them escapes, they take two of the pages with them. Later, Sam and Dean bring the journal to Bobby who discovers that the journal is an instruction manual to open a door to Purgatory. He discovers that it can be used to let something in from Purgatory, but all he is able to determine is that the creature they plan to release is named the "Mother of All."
The dragons are later seen using the pages to release Eve from Purgatory. They sacrifice a virgin by throwing her into a chasm in a cave to be used as a vessel for Eve to manifest herself.
Sam takes Dean and Bobby to the Campbell Compound to search for a way to kill Eve. They discover a hidden library beneath Samuel's office that contains everything he knows about the supernatural. When they are researching the lore books, Bobby can be seen reviewing the journal searching for a way to kill Eve. They eventually learn that Eve can be killed by the ashes of a phoenix and that one was killed by the Colt in Sunrise, Wyoming, back in March 5th, 1861. After Castiel takes Sam and Dean back in time, he gets ambushed by Rachel and the resulting battle leaves him weakened and Rachel dead. Bobby is seen reviewing and translating the journal when Castiel appears.
The Dragon's Journal can be seen on Bobby's desk when Sam and Dean are reviewing Samuel's journals to discover how Castiel and Crowley are going to open Purgatory.