|Out of universe information|
Thalan's people are an extinct civilization that was wiped out by Phoebus' people presumably several thousand years ago.
After being brought to Atlantis in 2006, Thalan and Phoebus sought to continue this war for their people by taking control of both Col. John Sheppard and Dr. Elizabeth Weir respectively in hopes of finally ending their long battle. However, both sides eventually lost as the bodies they had taken control later convulsed, resulting in both alien consciousness being expelled completely which meant that they had failed to win the war. (SGA: "The Long Goodbye")