In the view of Dio Cassius , a contemporary observer , his accession marked the descent `` from a kingdom of gold to one of rust and iron '' . The historian Edward Gibbon took Commodus 's reign as the beginning of the decline of the Roman Empire . In the view of Dio Cassius , a contemporary observer , his accession marked the descent `` from a kingdom of gold to one of rust and iron '' â '' a famous comment which has led some historians , notably Edward Gibbon , to take Commodus 's reign as the beginning of the decline of the Roman Empire .