Stargate/The Ancients

The Alterans, or Ancients as they are more commonly known, are the race that built the original network of Stargates throughout several galaxies, as well as apparently seeding life in the Milky Way galaxy and leaving their technology liberally scattered around to cause trouble millennia after their departure. The Ancient race was originally quite similar to modern humans in form, although they possessed advanced healing abilities, extremely long life spans, and highly advanced technology.

At the climax of a long, bloody conflict with the Wraith in the Pegasus galaxy during which the Ancients slowly lost most of their holdings to attrition, the last surviving Ancients gave up, sank their last remaining city-ship (Atlantis), and returned to Earth via the Stargate. Eventually, the surviving Ancients ascended to a higher plane of existence where they currently reside, refusing to get involved in the affairs of lower life forms even when those affairs can be directly faulted to the pre-ascension Ancients (such as the Wraith in the Pegasus galaxy, who are the result of an accident caused by the Ancients, and the numerous times in the Milky Way galaxy that Goa'uld or other forces have acquired abandoned Ancient technology and used it for evil).