The Archive of Islos was guarded by the Archivians. According to the Chief Archivian, it was the most precious resource in the sector. It had a very large door. (WC: The Archive of Islos)
When Islos was colonised in the Dark Times, it became home to a small populations of Magnoxans who survived the destruction of Magnox, which was once a great receptacle of knowledge. Peschell became the "Oracle of Islos" in the tradition of the Minds of Magnox, and established within her home a great library of books as well as a digital archive. The Eleventh Doctor retrieved a data drive from the ruins of the Chronicon, containing all of Magnoxan history and 3% of all knowledge in the Dark Times, and donated it to Peschell's archive. This was early in Islos' history, and the planet would eventually achieve a "great future". (AUDIO: The Minds of Magnox)
In the human era, the Archive of Islos contained information on the Dark Times, the Great Vampires, the Kotturuh, the Lesser Order of Oberon, Mordeela, the Hond, and the Record of Rassilon. A Data Worm broke into the Archive and was trapped within its systems by the Archivians until the Archive was invaded by the Daleks. (PROSE: Secrets of Time Lord Victorious)
The Dalek Emperor decided to invade the Archive of Islos for historical information to clarify a message it received from a Dalek Drone. (PROSE: The Last Message) When the Daleks came for the archive, the inhabitants of Islos abandoned the archive, leaving the Archivians to protect it. The Chief Archivian refused to surrender the archive to the Daleks and used the information within it to make contact with an Entity from outside time and space. The Entity agreed to transport the surviving population and the contents of the archive to safety, in return for the Daleks. When the Daleks captured an empty building, they exterminated all the Archivians, except for the Chief Archivian, who revealed their bargain as the Entity emerged through a portal. (WC: The Archive of Islos)