Complex sentence: The setting comes about 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, and the alien race known as the KessRith conquer most human worlds and eslave them.
Split into simpler sentences: The setting comes about 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 conquer most human worlds and eslave them.