The Iconians were an ancient and highly advanced civilization that was presumably 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")
Later that year, the territory of the Iconians in the Beta Quadrant was labeled on the star chart "Alpha/Beta Quadrant Overview" in the ready room aboard the USS Discovery. (DIS: "Magic to Make the Sanest Man Go Mad")
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.
The Iconians appear as the main antagonists of a major story arc of Star Trek Online, culminating in their waging a war against an alliance of powers from all four quadrants.