Complex sentence: Sienna later explains that Zobrist was a geneticist who advocated the halting of humanity's growth, due to its out of control population and that he was rumored to be working on a means to do so using an engineered disease.
Split into simpler sentences: Sienna later explains that Zobrist was a geneticist who advocated the halting of humanity's growth, due to its out of control population. And that he was rumored to be working on a means to do so using an engineered disease.