Iconia was the homeworld of the ancient Iconians, whose surviving technology nearly destroyed the Enterprise and a Romulan warbird.
Iconia was the world of the Iconians, an ancient and technologically advanced people who were believed to be little more than a legend. Their civilisation had been destroyed long ago, but in Contagion the ruins of the planet were found inside the Romulan Neutral Zone.
Captain Donald Varley of the USS Yamato believed he had located the planet. A probe launched from Iconia sent a program that took over the Yamato's systems and destroyed the ship, and the same program then spread to the Enterprise and a Romulan warbird.
Picard led an away team to the surface and found an Iconian gateway, a device able to open portals to other worlds. To keep the technology from falling into anyone's hands, he destroyed the control centre and escaped through the gateway to the Enterprise. Iconian gateways surfaced again later in Star Trek history.