Inhabitants of M5S-224
Intelligent non-corporeal entities who inhabit much, if not all, of the surface of M5S-224. To the outside observer the entities appear to be nothing of sentience, but rather a dense fog (though this is an inaccurate term as there is no water vapor). The creatures generate a tremendous energy field which the Stargate is capable of converting into fuel to power wormholes.
When the gate activates lives are lost. The creatures deem the sacrifice of those who are used to power the gate locally to be acceptable losses to prevent trespassers from discovering the fog's true identity. However, dialing to another galaxy would kill millions of the beings, which the mist aliens find to be too great a cost.
To protect themselves, the mist aliens are capable of rendering those who would use the Stargate in such a way unconscious, occupying their minds in fantasies until they expire on the planet surface.
The entities are not without reason, however. They are true to their word and will permit the small loss of their own lives to allow individuals to reactivate the Stargate and depart for another local gate.
Home - Worried that the Atlantis expedition's resolve to return home is great enough to devastate their society, the misty inhabitants of M5S-224 decide to render Weir and Sheppard's team unconscious, not caring that they will soon die without sustenance.