The Dewans were a humanoid species native to Dewa III, known as New Romulus in the 25th century. In 200,000 BCE, the Dewans were a servitor race of the Iconians, who constructed an Iconian gateways on the planet. (STO mission: "Secrets of the Ancients")
The Dewan civilization rose and fell two times. Nothing is known about the first iteration other than that it was centered around serving its Iconian masters. The Iconians held absolute control of the planet and quelled rebellion with extreme prejudice.
Following the post-apocalyptic horror, it took the Dewans 50,000 years to rebuild their civilization. The technological and scientific knowledge of the Iconians had been lost, and the Dewans now considered the Iconians as gods that had walked among them.
The Dewans began reach out for space and built the first ships to explore Dewa's orbit. Five years later, however, they found an Iconian gateway underground. Space travel was scuttled and resources were pooled into cultivating geothermal energy and reactivating the gateway instead. The reactivation was a catastrophic failure and the planet entered another post-apocalyptic winter. Although the survivors of the cataclysm managed to hold out for some time in caves, the radiation was much more persistent than 50,000 years earlier, and the Dewans went extinct. (STO mission: "Research History of New Romulus")
Dewan artifacts remained on the surface and could be found beneath the ruins of the proto-Romulans settling the planet in the 4th century. (STO missions: "New Romulus: Hwael Ruins", "Research History of New Romulus"
200,000 years before 2409, the Dewans served the Iconian Empire. Millions of Iconians and Dewans lived on the planet. When the subjugated of peoples of the Empire rose up against their technologically superior masters, they bombarded Dewa III from orbit. The Iconians fled and the Dewan civilization was destroyed by the tectonic and ecological cataclysm induced by the bombing. A continent with three million Iconian and Dewan inhabitants was scoured.
After the radiation abated, the Dewans began to rebuild over the course of 50,000 years, finally achieving rudimentary space travel. Five years after that achievement, the efforts were abandoned in favor of reactivating a recovered Iconian gateway. A geothermal power plant was build around it. During the activation, the planet suffered another cataclysm, the tectonic activity crushing the Dewan civilization once more, and the radiation killing of the survivors that had retreated to the caves. The last Dewan died 150,000 years before 2409.
The universe forgot about them until the Romulan Republic erected claimed the planet as its capital in 2409. (STO mission: "Research History of New Romulus") The Romulans showed archaeological interest in the Dewan and proto-Romulan cultures that had existed on the planet 150,000 an 2,000 years earlier, respectively. (STO mission: "New Romulus: Hwael Ruins") The gateway and the power planet were rediscovered by the Tholian Assembly. After they had been driven out by the Romulans, the process began to reactivate the gateway and find out more about the Dewans and their Iconian masters. Among the findings was a holographic recording providing the first visual footage of a Dewan. (STO missions: "The Power Source", "Secrets of the Ancients")