The Arnor, also called the Precursors, was an ancient spacefaring civilization that is thought to have inhabited most of the galaxy during its height and had long since disappeared.
The Arnor suffered a schism and fell into civil war. One side, known as the Dread Lords (an Iconian name associated with the faction), was on the brink of victory before they were sealed off in a pocket universe. The few surviving Arnor simply disappeared.
While few races had any dealings with the Arnor during the colonization of the galaxy following the invention of the Hyperdrive signs of the Arnor were found in ruins on their formerly inhabited worlds and through artifacts and derelict starships scattered throughout the galaxy. Despite the technology being ancient, it was tremendously powerful and was widely sought for by the factions colonizing the galaxy.
The Drengin Empire accidentally released the Dread Lords from their prison using an artifact, resulting in the Dread Lord War. Another Precursor device was found by the Terran Alliance and used to place a shield around Earth to protect it from invasion.
The Altarians also claimed to have found several Precursor "doomsday weapons" and threatened to unleash them in the event of invasion of their homeworld.
At least one Arnor is known to have survived into the 23rd century. This Arnor helped the Terran Alliance finally defeat the Dread Lords by developing the Terror Star, a weapon capable of destroying an entire star system. Following the end of the Dread Lord threat the Arnor disappeared. His fate or that of any other Arnor is unknown.