Cassius Dio was a Roman historian and senator who lived from around 155 to 235 AD. He is best known for his work, the "Roman History," which covers the history of Rome from its legendary founding to the reign of Emperor Severus Alexander. His writings provide valuable insights into Roman politics, society, and military affairs. Dio was born in Nicaea, in what is now Turkey, and served as a consul in Rome. His historical accounts are notable for their detailed narratives and critical analysis, making him an important source for understanding the complexities of the Roman Empire during his time.