Keith R. A. DeCandido 
Gateways #4 [EPUB ebook] 
Demons of Air and Darkness

Once they moved from world to world in a single step, through innumerable doors that spanned the galaxy. They were masters of space, and to those who feared them, they were demons of air and darkness. But long ago they left their empire and their miraculous technology behind. Now someone has found the key to it, and all those doors have been ung open. A world near Deep Space 9, threatened with destruction from the distant Delta Quadrant, becomes the focus of a massive rescue effort as Colonel Kira Nerys, her crew, and some unexpected allies ght to avert disaster on a planetary scale. Meanwhile, as Lieutenant Nog and Ensign Thirishar ch’Thane search for a way to shut down the spatial portals forever, Quark becomes involved in a dangerous game that could determine, once and for all, who will control the Gateways.