The Eternal Ones are a race of god-like, extra-dimensional beings who consume all sentient life in the Universe from time to time. According to the Daktaklakpak, the Eternal Ones' name comes from their DNA formulae that start as 'Eternal1'. Their complete name is their complete DNA formulae, something so complex that any sentience that tries to learn it all, organic or machine, becomes insane.
The Eternal Ones evolved from sentient organic beings until they lost their material bodies and became pure energy, reaching the verge of transforming into "something else". The Eternal Ones were afraid to evolve and lose their last physical vestige, and made a plan to stop their evolution that required consuming incredible quantities of sentient energy. To harvest sentience in great quantity, the Eternal Ones planted "seeds" throughout the universe that created all the other races in the Star Control universe.
The Precursors became aware of this and devolved themselves to non-sentient animals and created semi-sentient machines called Daktaklakpak to re-evolve themselves when the harvest was over. Unfortunately the Daktaklakpak forgot their main objective and started to wander the galaxy collecting and protecting all Precursor technologies. The long-term purpose for the Precursors was to develop an artifact to optimize the harvest for the Eternal Ones. The Precursors had noticed that sentient harvest was incredibly inefficient, and theorized that by optimizing the harvest, there would not be a need for such genocide.
In Star Control 3, the player, as the human leader of the sentient races, foresees the harvest, and unites most races to find a solution. He finds the unfinished artifact and re-evolves a Precursor who helps the player finish it in time for the next harvest. The much greater efficiency of energy absorption allowed by the artifact allows the Eternal Ones to feed without requiring the death of all sentient creatures.