In Galactic Civilizations, there are many allusions to an enigmatic entity called the Precursors. They are a handfull of hyper-advanced space faring civilizations who for unknown reasons are no longer present.
There are two known Precursor Races, the noble Arnor and the evil Dread Lords. The Arnor are a peaceful race, whose actions and powers are possibly responsible for the galaxy's evolution. The Dread Lords, the polar opposite of the Arnor, are a race of warmongers that sought galactic conquest. The two races had fought a desperate civil war prior to the events of the game.
The Precursors have a major role in Galactic Civilizations 2: Dread Lords and it's expansion, Twilight of the Arnor.