The Mysterium Xarxes was a tome written by Mehrunes Dagon, who scribed it in "the deserts of rust and wounds". It was an artifact of great - and evil - power. The book was dangerous to handle, as even reading from it required magical protection from its power.
The book was given by Dagon to Mankar Camoran. After studying the tome, Mankar wrote the Commentaries on the Mysterium Xarxes (also known as the Mythic Dawn Commentaries). Inspired by the prophecies and promises within the book, Mankar founded the Mythic Dawn, a Daedric cult which worshipped Mehrunes Dagon. The Xarxes acted as the cult's holy book, and was stored in the Mythic Dawn's hidden shrine in the caverns beneath Lake Arrius in Cyrodiil. Using the power of the book, Mankar created Gaiar Alata, or "Paradise", an alternate realm where the souls of Mythic Dawn cultists went in death.
In 3E 433, following the Mythic Dawn's assassination of Emperor Uriel Septim VII and all of his legitimate heirs, the Blades infiltrated the cult and stole the Mysterium Xarxes from their shrine to Mehrunes Dagon. It was taken to Cloud Ruler Temple, where Martin Septim, the bastard son of Uriel and the heir to the throne, translated the Xarxes and discovered a way to create a portal to Camoran's Paradise. The ritual required a Great Welkynd Stone, a Great Sigil Stone, a Daedric artifact, and an Aedric artifact. The Hero of Kvatch entered Gaiar Alata and defeated Camoran, retrieving the Amulet of Kings. The Xarxes was destroyed in the process.
A single burned page from the tome survived the events of the Oblivion Crisis, and came into the possession of the Vesuius family, who were once members of the Mythic Dawn. In 4E 201, the page was put on display by Silus Vesuius in his "Mythic Dawn Museum" in the Skyrim city of Dawnstar.