From Memory Alpha
The Iconians were an ancient and highly advanced civilization that was destroyed some 200,000 years ago by the orbital bombardment of their homeworld, Iconia. Ancient texts called the Iconians "Demons of Air and Darkness", who were said to have the ability to appear at will on far-flung planets without the use of starships. Later this was found to be a reference to the Iconians' gateway technology, whose instant teleportation allowed them to establish outposts at least as far as Vandros IV in the Gamma Quadrant. Until the discovery of Iconia in 2365, the Iconians were still dismissed by many as nothing more than myth.
Historically the Iconians were generally described as a race of conquerors, though recently some scholars disputed this claim as a distortion perpetuated by species who feared and demonized their advanced technology, and eventually destroyed them. It was speculated that not all of the Iconians perished in the attacks, but rather used their gateways to escape to other worlds. Indeed, the similarity between the Iconian language and the later languages of Dewan, Dinasian, and Iccobar has been cited as evidence of this theory. (TNG: "Contagion"; DS9: "To the Death")