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. This was later 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 by the late 24th century, some scholars had 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 was 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 B'Elanna Torres how gullible Kim was. (VOY: "Inside Man")
In 3190, Federation Security identified "surviving members of the Iconian Empire" as one of the civilizations that may have technology advanced enough to create the Dark Matter Anomaly. (DIS: "The Examples")
According to Terry J. Erdmann's Star Trek: Deep Space Nine Companion (pp. 348-349), 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. The former Iconian civilization was an advanced but peaceful galactic superpower, expanding through the use of gateway technology and guarded by an engineered "Herald" servitor species. An alliance of lesser powers formed over fear and resentment of Iconian hegemony bombarded Iconia and its colony worlds from orbit, and leaving only a small number of Iconian survivors. The near-genocide, coupled with the loss of a collective archive called the World Heart, traumatized and radicalized the survivors. Converting themselves into energy-based lifeforms and relocating to a Dyson sphere located in the Andromeda Galaxy, the Iconians spent the next 200,000 years planning their return to the Milky Way. They developed their Heralds into a powerful military, and acted through agents to destabilize civilizations in all four galactic quadrants.
Iconian action was directly or indirectly responsible for a number of incidents throughout the 24th and 25th centuries. They worked with the Tal Shiar, whose experiments with Iconian gateway technology led to the supernova that destroyed the Romulan homeworld. The Elachi, a species native to the Mycelial plane, were suborned as a servitor race and used to kidnap and scan Humans. The Solanae constructed the Iconian Dyson sphere habitats, and after moving to subspace, were also used in kidnappings and experimentation. The Iconians attempted to take control of the Federation by controlling key figures through bluegills infestation, and by the same means successfully captured the leadership of the Vaadwaur, rebuilding their military and unleashing them on other Delta Quadrant powers. They manipulated the Undine into attacking the Alpha and Beta Quadrant before appearing directly, commanding their Heralds in a war against forces from all four galactic quadrants. Eventually the Iconians were convinced to stand down and declare an armistice, retreating to their Dyson sphere, which was teleported from Andromeda to encompass the Iconian system.
The game also lists its Borg designation as "Species 29".