They fell in battle against Xaxion, but not after destroying his physical form.
The essence of Xostheron and Akaerna-Sagai was captured by these men and women, who were inspired by it and gained knowledge of the future. This knowledge manifested itself over time and was written down in the form of the Prophecies. Eventually, a rift formed in the ranks of the Archivists over what to do with the Prophecies. Those that chose to exploit the knowledge to use it to break history became the Chancellors. The rest remained Archivists, and converted to the Holy Light, seeking refuge in neutral Dalaran.
Eventually, the regime of Javali saw the Hareveim become the influential force in Hesperia. The Hareveim desired the Prophecies, and used Javali to disband the Archivists. In the meanwhile, the sixth member of the Council of Six, Councillor Emberstone had been trying to get his hands on the Prophecies, and had helped to see Javali gain power so that the Archivists would be disbanded.
He failed and was forced to flee into Alterac, and Javali sent Lucio Benado to track him down and spy on operations in Alterac.
At Ythan'alai, Meryl Winterstorm revealed that he was one of the founders of the Archivists, though he had splintered off to pursue his own agenda, and had taken on the alias of Lord Giren to spy on the Chancellors in Alterac and manipulate their designs.