Destruction under the Mongol Empire
The Mongol conquests resulted in widespread and well-documented death and destruction throughout Eurasia, as the Mongol army invaded hundreds of cities and killed millions of people. As such, the Mongol Empire, which remains the largest contiguous polity ever to have existed, is regarded as having perpetrated some of the deadliest acts of mass killing in human history.
Source: Wikipedia — Destruction under the Mongol Empire (CC BY-SA 4.0)