Complex sentence: The rebellion not only posed the most serious threat towards Qing rulers, it has also been called the "bloodiest civil war of all time", in which between 20 and 30 million people died during its fourteen-year course from 1850 to 1864.
Split into simpler sentences: The rebellion not only posed the most serious threat towards Qing rulers; it has also been called the "bloodiest civil war of all time". Between 20 and 30 million people died during its fourteen-year course from 1850 to 1864.