Complex sentence: Before leaving however, they had established the Lizardman empire (ruled over by the Slann) as their servants, and had created the races of Elves, Dwarfs, Humans, Ogres and Halflings.
Split into simpler sentences: Before leaving however, they had established the Lizardman empire (ruled over by the Slann) as their servants. In addition they had created the races of Elves, Dwarfs, Humans, Ogres and Halflings.