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)
Despite legends describing the Iconian Empire as brutal and oppressive, it used its technology to benevolent ways, led by M'Tara, the Iconian people was known for their openness and hospitality. However, suspicion and fear motivated several species from all over the galaxy to unite against the Iconian Empire. These species formed an alliance, which 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 - What Lay Beyond short story: "In the Queue")
A captain from the year 2410, joined by the Klingon Captain Kagran and disgraced Romulan Empress Sela, arrived during this time from a time portal and learned the truth of the Iconian Empire's fall - they were a benevolent race that was highly technological, though a bit arrogant. Many races coveted their technology, seeking to use it for conquest or being about to eradicate disease without knowing of the consequences of using such high technology. These races united and attacked Iconia's colony worlds before attacking Iconia itself. The trio aided three Iconians - M'Tara, L'Miren and T'Ket - in evacuating the Iconians to Dewa III. However, Sela, wanting to complete the trio's original plan to kill the Iconians and stop the war that was ravaging their world at the time, struck killing a number of Iconians and crippling L'Miren, leading to T'Ket to vow to eradicate the Romulans for her treachery. (STO mission: "Midnight")
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 Iconians had several servitor races which served them in order to complete their common goals. In the early 2410, a Dyson 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.
During the Alpha Quadrant Alliance's assault on Iconia, M'Tara was killed by enemy forces and L'Miren was chosen to replace her as the empire's leader. After the time travelers returned to the present, they hailed L'Miren's flagship and returned the World Heart to her people. Realizing they were the ones who saved her from death millenia ago, L'Miren stands down and withdraw to Iconia, asking only to be left alone so they could rebuild, contemplate and grieve. (STO - The Iconian War mission: "Uneasy Allies", TNG novel: The Devil's Heart)