The Iconian Empire was a massive interstellar empire that once existed in the Milky Way Galaxy. It is known that this entity had fallen about 200,000 years ago.
Rise and Fall of the Empire
The ascendancy of the Iconian race began thousands of years ago when their race made use of the Dream Gem, to develop their gateway technology. Using this incredibly advanced technology, they formed the Iconian Empire which by 200,000 years ago is known to have reached all four quadrants of the galaxy. (TNG novel: The Devil's Heart)
It is believed that the Iconian Empire used its technology to benevolent ways. However, suspicion and fear motivated several species from all over the galaxy to unite against the Iconian Empire. These species formed an alliance, wich journeyed to the Empire's capital planet and homeworld of Iconia, where they bombarded its surface, where it was believed that the "Demons of Air and Darkness", as other species called them, were finally destroyed. In over 200,000 years no other state had achieved the same level of greatness as the Iconian Empire. (TNG novel: The Devil's Heart, VOY short story: "In the Queue")
Although the Iconians were believed to be extinct, some used the Dream Gem to activate their gateway network in order to survive. In the early 25th century, the Herald Sphere suddenly jumped from the Andromeda Galaxy to some location near the Iconia system, bringing the Herald fleets, signaling the beginning of their invasion. These events led to what became known as the Iconian War. (STO - The Iconian War mission: "Uneasy Allies", TNG novel: The Devil's Heart)