Promellians were a humanoid civilization native to somewhere in the galaxy's Alpha or Beta Quadrants. At their height, the Promellians were warp-capable, technologically advanced space-farers. By the 24th century, the Promellians were extinct, the result of a decline begun a thousand years prior.
A key moment in Promellian history was their first contact with the Menthar species. A Promellian ship unknowingly intruded into a star system considered sacred by the Menthar, and came under attack by Menthar forces. The Promellian ship was destroyed, though not before destroying two of its attackers. Both sides declared war soon afterwards. (TLE novel: The Buried Age)
The war between the Promellians and the Menthar resulted in the mutual extinction of both races. The final battle was fought in the remains of the planet Orelious IX, in the late 14th century. (TNG episode: "Booby Trap")