The Thallonian Empire was an extremely xenophobic society located in Sector 221-G of the Beta Quadrant, located between Federation starbase Deep Space 5 and the Romulan Star Empire. (Star Trek: New Frontier)
For centuries the empire was ruled over by a royal family who were based on the Thallonian homeworld, Thallon. By the mid-24th century numerous civilizations, including the Nelkarites, had been conquered and forced to become subjects of the empire. Naturally, the Thallonians were extremely protective of their territory, and any unauthorized trespassers that were caught inside their space were sentenced to death.
The Empire finally collapsed in 2373 following a coup d'etat, which had been engineered by the neighboring Danteri, and had left the majority of the royal family. Following the break down of law and order, the collapse of the Empire was swift. Hundreds of thousands of refugees poured out of Thallonian space into neighboring Federation territory. (NF novel: House of Cards)
In the late 2370s, the sole survivor of the Thallonian royal family, Si Cwan, drew many of the former subject worlds together to form the democratic New Thallonian Protectorate. (NF novel: After the Fall)