Imperium is one of the two ancient transendental entities, the other being Pax, which fight for dominance since the beginning of time. Both entities influence species and cultures which emerge in the universe to fight for them. Nobody knows the true form of these beings. It is said that they are beyond the understanding of any sapient species in the universe.
Imperium represents hierarchy, authority, oppression and war. Their goal is to bring those values to the whole universe. Species on their side tend to be authoritarian hierarchical empires. Often these empires turn into oppressive regimes.
One of the species serving them in the cosmic conflict are the Arcturians.