An ancient civilisation destroyed some 200,000 years ago whose technology, including gateways that link distant places, still proves dangerous to those who find it.
The Iconians were an ancient and powerful civilisation that dominated a large area of the galaxy roughly two hundred thousand years ago. Their enemies called them Demons of Air and Darkness, and legends held that they could appear anywhere without a ship. Their homeworld, Iconia, was laid waste by orbital bombardment, and the empire disappeared.
In 2365, in the episode Contagion, the USS Yamato finds Iconia inside the Romulan Neutral Zone and is destroyed after an Iconian probe infects its computer with software that overwrites its systems. The same software then spreads to the Enterprise-D and to a Romulan warbird. Jean-Luc Picard, Data and Worf reach the planet and find a working gateway that can open portals to other worlds, before Picard destroys the facility to keep it out of Romulan hands.
Iconian gateways reappear in Deep Space Nine, where Jem'Hadar renegades try to seize one in the episode To the Death. The Iconians embody the theme of a lost advanced people whose legacy outlasts them.