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 Species 8472 and the Tholians to adopt aggressive postures towards the Klingon Empire and the Federation.