The Promellians were a warp-capable technologically-advanced humanoid species. This species destroyed themselves alongside the Menthars, as a consequence of their long war with each other. The war ended with the battle of Orelious IX in the late 14th century.
The Promellians operated advanced battle cruisers which operated at a technological level that rated as rather crude in terms of 24th century Federation technology, but were highly advanced for their time. They also used a form of information storage device called memory coils, which were a crude analog of a 24th century Federation isolinear optical chips. Their Lang Cycle fusion engines were also a well known form of propulsion technology. (TNG: "Booby Trap")
While the appearance of the Promellians were not described in the script, it is of note that all of, or portions of, the Promellian makeup design was later for the Algolians, Rotciv, and Yint's species.
In scripted dialogue between Jean-Luc Picard and Data, which was later cut, it was revealed that early Federation starship designs were influenced by the Promellian design innovations. As an example, it took four attempts at tactical command design before the Federation was comparable, in the 23rd century, to the sophistication of the Promellian tactical display. Further references included that to their use of positron ducts that channeled power from one end of the ship to the other.