The Warden Eternal was created to protect the Domain, the Foreunner’s primary repository for knowledge. Following the New Phoenix Incident and the destruction of the Ur-Didact’s ship Mantle’s Approach in 2557, a portion of the ship and Cortana were transported via slipspace to Genesis, the an artificial world and the physical entrance to the Domain. Cortana managed to deceive the Warden Eternal and gained access to the Domain, using it to cure her rampancy and take control over the Warden.
In October 2558, after Cortana began waking Guardians to begin the Rule of the Created, the Warden was sent to defend against Fireteam Osiris and later Blue Team from gaining access to Genesis and the Domain. He was eventually overpowered by Cortana, however, as she planned to imprison Blue Team until her Created took full control of the galaxy.