Demons of air and darkness - Keith R. A. DeCandido

Once they moved from world to world in a single step, through innumerable transdimensional gateways that spanned the galaxy. They were masters of space, and to those that feared them, they were demons of air and darkness. But long ago they left their empire and their miraculous technology behind. Now the key has been rediscovered, and all the doors have been flung open. Open for exploration... or open for invasion.
On a world near space station Deep Space Nine, Colonel Kira Nerys, her crew, and some unexpected allies fight to avert destruction on a planetary scale. Meanwhile Lieutenant Nog and Ensign Thirishar ch'Thane search for a way to shut down the portals forever; and behind the scenes Quark plays a dangerous game that could determine, once and for all, who wins control of the Gateways...