The Precursors are the beings that roamed the galaxy thousands of years before the younger civilizations had left their homeworlds. The main precursors (that are known) are the Arnor and the Dread Lords.
The Arnor and Dread Lords were created by the Mithrilar to populate the Universe which they had just made. In their creation the Mithrilar decided create a fundamental difference between the Arnor and the Dread Lords, the main difference being that the Arnor understood time and the Dread Lords perceived it.
To clarify, an Arnor would be happy to sit and watch a glacier, seeing it slowly change and shape the land as it moved without caring how much time was passing or what was going on in the world around it while a Dread Lord would find the experience completely pointless.
Because the Dread Lords were the first beings to comprehend finite existence and see that even though they may not be able to grow old and die, the universe would one day come to an end, they were gripped with a drive to explore, discover and conquer.
During this period of expansion the Dread Lords discovered many things, unknown dimensions, parallel universes and even a pocket universe (which ironically, they would later be trapped in.) But the discovery that would have the most effect upon them was the realization that other species had begun to populate the galaxy.
This news disturbed the Dread Lords greatly, for where the Arnor saw a new and fascinating subject to study, the Dread Lords saw a threat.
What was at first a philosophical difference between the two grew into an all out civil war between the Precursors to decide the fate of the newer races.
The Dread Lords innately being the stronger of the two defeated the Arnor in battle after battle, until only the Arnor's homeworld was left.
Eager to destroy the last obstacle in the way of them the Dread Lords began their assault on the Arnor homeworld, but this time, the Arnor were ready.
For they had learned of the existence of the Dread Lords' pocket universe and constructed a device that would seal the Dread Lords inside.
This level of cunning was completely unexpected and the Dread Lords were caught completely offguard and were sucked straight into the pocket universe.
The Arnor had achieved victory against the Dread lords but it had cost them dearly. They had become a broken people and at the advent of "Twilight of the Arnor" there is only one known to exist in all of the known universe.