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")
In 2377, Tom Paris told Harry Kim he received a call from an "Iconian scientist" who said he had a trans-dimensional gateway that could take USS Voyager anywhere in the galaxy. In reality, Paris was just saying this to prove to Torres how gullible Kim was. (VOY: "Inside Man")
According to Terry J. Erdmann's Star Trek: Deep Space Nine Companion, the writers of the episode "To the Death" mistakenly referred to the Iconians as the Tkon, causing the art department to design and produce sets based on graphics seen in "The Last Outpost". The error was discovered not long before shooting took place and corrected.
Taking place in 2409, the computer game Star Trek Online develops the Iconians as the ultimate villains. The game depicts them disturbing the Alpha Quadrant through various actions, e.g., being behind the destruction of Romulus and causing factions like the Undine (Species 8472) and the Tholians to adopt aggressive postures towards the Klingon Empire and the Federation. It is also revealed that the Iconians have many "servitor races" such as the Elachi and the Solanae, who built a Dyson sphere in the Delta Quadrant. An Iconian is seen for the first time in 2410 during the STO mission 'Surface Tension'. An Iconian shows up when the Federation and Klingon Empire are about to end their war, warning the powers to not attract their attention again, and leaving through a form of gateway after killing the members of the Klingon High Council with the exception of the Chancellor. Admiral Tuvok believes this attack occurred to prevent another alliance against their Empire, the first one destroying them 200,000 years earlier. In the Delta Rising expansion's bonus episode "What's Left Behind," it's revealed that they are behind the leadership of the Vaadwaur being under the control of a parasite organism known as the "Bluegill" in exchange for them to become a feared power in the Delta Quadrant. This would also mean that the Iconians were behind the attempted takeover of the Federation in the mid-24th century. Soon afterward, they teleport a Dyson sphere from the Andromeda galaxy to a point near Iconia, signalling the beginning of their invasion.