Complex sentence: After a massive plague decimates or completely depopulates the human race on many worlds of the Orion arm of the Milky Way galaxy, which includes Earth/Terra Firma, an alien race known as the KessRith conquers most of the human race and enslaves them.
Split into simpler sentences: After a massive plague decimates or even completely depopulates the human race from many worlds of the Orion arm of the Milky Way galaxy, which includes Earth/Terra Firma, an alien race known as the KessRith attack. The KessRith conquers most of the human race and enslaves them.