The Chief Archivian was the leader of the Archivians, a race of robots who had been built to protect the Archive of Islos at all costs, considering it to be the most precious resource in the sector.
When the Daleks of the Restoration Empire attacked Islos, the Chief Archivian was called by the Emperor of the Restoration. They refused to surrender the archive even though the planet was left defenceless after the Orbiting weapons platforms were taken down. The Chief Archivian surrendered to the Daleks only after the surviving population had gone through the portal along with the contents of the archive. Outraged, the Daleks exterminated every Archivian, except for the Chief Archivian. When the Emperor asked the Chief Archivian what did they offer the race that saved the inhabitants of Islos in return, they responded, "You", revealing to the Daleks that they had been offered up by the Chief Archivian and that entering the Archive had become a trap. (WC: The Archive of Islos) The Dalek were quickly driven off of Islos by the Entity. (WC: The Sentinel of the Fifth Galaxy)