Complex sentence: There, the villain turns out to be Don Onion, leader of the Tabemon monsters and a former colleague of the Great Elder, who began corrupting their previous experiments to take over the world.
Split into simpler sentences: There, the villain turns out to be Don Onion, a Tabemon monster who leads the entire race. It is stated that he was a former colleague of the Great Elder, who began corrupting their previous experiments to take over the world.