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
Until the discovery of Iconia in 2365, the Iconians were still dismissed by many as nothing more than myth.
The Discovery of Iconia in the Romulan Neutral Zone was marred by tragedy when the Galaxy Class USS Yamato was destroyed by infection with Iconian Programming. The canon information about the Iconians can be found here .
The El-Aurians knew of the Iconians. A faction of El-Aurians black mailed Starfleet into putting them on the last known Iconian outpost, this is the long term arc of the USS Discovery in ST-OM.
El-Aurian programmer Deena Foote was able to use Iconian software patterns to completely defeat Starfleet Security on the USS Discovery. In truth Deena was the project lead of several El-Aurians who created this software, but she gracefully took the blame, and refused to reveal who else was involved.
In the Epiphany Trek setting the Iconians are organically natives of Omicron Ares 4. The location I set the events of Heinlein's Stranger in a Strange land. Events that came to a head in the early 2100s the first and Last time the Grokites have been in the news. "Iconia" was the world the rebels based themselves off of. the Old Ones would never have allowed if from the real home world.
The El-Aurian attempt at colonizing Howard was a failure, due to unforeseen problems with wildlife. Starfleet took charge of the Iconian Gateway on Howard 223, and it's used as a reverse engineering project and as a highly classified transporter with infinite range.
The USS Sigma Seven encountered in Iconaian (Aresian) Old One in the Onthed Alliance area. It was guarding ancient technology and aiding a group of natives.
It is known they contented with the Rishans. We don't want to know what that looked like.
The Iconians had the ability to make portal to distant location, bypassing travel between. The mechanisms of this are being studied. The Iconian Gateway, like the Transporter requires no receiver on the destination side. The gateway is invisble but present on the destination side. The destination is visible through the gateway mechanism.
The Iconian gateways seem similar in many ways to the Inverter used by the Ansata Rebellion on Rutia , however it is much more advanced and deoes not seem to have the side effects that the Ansata machine does.