This species ran rampant like a plague through the Pegasus Galaxy, feeding on the life-force of the human inhabitants seeded by the Ancients on many worlds, until only Atlantis remained untouched. The city fell under siege for an extended period of time until the Ancients, defeated, decided to submerge it, the survivors returning to Earth to live out their lives in peace. The Wraith went into hibernation until a substantial population could be found that would allow all of them to thrive once more.
The threat of the Wraith has remained a dark cloud over the sparsely populated human worlds in Pegasus -- but the galaxy also offers great hope and potential to the eager explorers from Earth.
Rising, Part 1 - Dr. Daniel Jackson discovers that the Ancients' lost city resides in the distant Pegasus Galaxy, and encourages a team to be sent to see if the Ancients still exist.
Midway - Earth scientists work to complete a space station positioned in the empty space between the Milky Way and Pegasus Galaxies, allowing for inter-galactic travel by Stargate.
Enemy At the Gate - After millions of years Atlantis departs the Pegasus Galaxy, traveling to Earth to defend it from a Wraith attack.