Fenrir was exiled as punishment for the destruction of a populated star system when he did an unauthorized deployment test of a Isa device he created. He was sentenced by the Asgard High Council to be put in stasis on his ship, the Aegis, which was then sent to roam the galactic clusters for five hundred seasons, on a course set to bring him back to the planet Hala when his sentence was over. His mind remained in a wakeful state so he could reflect on the mistakes his haste, arrogance, and belligerence had led to, prompting Fenrir to contemplate a return to his old research into helping the Asgard's genetic degradation rather than his military efforts due to the campaign against the Replicators. However, in a stroke of bad luck while traveling through the Pegasus galaxy, he found himself between the planet Heruun and a Wraith fleet preparing to cull its inhabitants when his ship dropped out of hyperspace to make a course correction. Fenrir and his vastly superior ship managed to win the ensuing battle, but took damage as a result and crash-landed on Heruun's moon.
Stranded, with no way to fully repair his ship without help, he repeatedly kidnapped Heruun locals and temporarily brainwashed them to fix his ship for a duration of weeks, later sending them back home with their memories of where they had been and done in that time period erased. As Fenrir was still trapped in his stasis chamber, he communicated through a hologram projection and the Risar, drone workers based on an earlier state of Asgard development that were more physically powerful than the Asgard while lacking their intellectual potential. During this time, he also captured a small number of Wraith when their ships attempted to attack Heruun, hoping that analysis of their genetic code might lead him to a discovery that would save his race. (SGA: "Nightfall")
This research ended when the Atlantis expedition came to Heruun and learned about Fenrir's true nature, prompting him to return captive Heruuni to their people as his experiments were causing long-term harm to their health due to the nanite 'tags' he used to mark his previous subjects, the tags building up in the victims' systems and damaging the brain's connection to the body. Although the expedition initially tried to conceal the death of his people from him out of fear at how Fenrir would react given his mythological counterpart's role in Ragnarok, the truth was revealed when a Wraith Hive ship contacted Fenrir after intercepting a transmission from Atlantis, also revealing the existence of the Asurans to try and convince him that Asgard and Wraith shared a common enemy.
Fortunately, despite his anger at this deception, Fenrir had already formed a bond with Teyla Emmagan, who sympathized with his plight due to the recent disappearance of her own people, allowing her to explain that they had simply concealed the truth to spare him the pain until they found the right time. Although Fenrir rejected the Wraith's further attempts to propose an alliance, the Wraith Queen sacrificed herself to destroy Fenrir with a bomb implanted in herself, Fenrir survived just long enough to beam the other expedition members back to his ship and provide Colonel Samantha Carter with access to the Aegis controls. The Atlantis expedition were able to use the Aegis to destroy the hive-ship at the cost of the Aegis itself, but were able to escape on an Asgard shuttle. After the expedition left the ship, it apparently vanished, prompting Sheppard and McKay to speculate that Fenrir's mind may have downloaded itself into the other ship and survived his body's death. (SGA: "Nightfall")