This is the Wraith. They are the new bad guys for Atlantis. This alien species feeds on the life
energy of humans. When the team use the new Gate network for the first time they stumble across one of their feeding grounds
so when the Wraith come to collect their herd some of the Atlantis people got captured.
When John Sheppard goes to rescue his people and the native people of that world he kills the
caretaker and accidentally wake the rest of the Wraith.