Genghis Khan founded the Mongol Empire,

Genghis Khan founded the Mongol Empire, which is recognized as being the largest contiguous land empire in history. Countless people were slaughtered in the course of Genghis Khan’s invasions.

John Man writes, "No culture before had wielded such destructive power as the Mongols; no culture had suffered as the Muslim world was about to suffer." 

According to a Mongol folk tale, adapted from Owen Lattimore, Mongol Journeys:

Where is Genghis Khan? He is not dead. What happened was this:

There was a king in a far country with a daughter fair as the sun. Genghis Khan asked for the maiden. The king secretly said to his daughter: Here is a knife, very small and very sharp. Hide it in your clothes, and when the time comes you know what to do. As Holy Genghis lay with her, she took out the knife and castrated him. Genghis cried out when he felt the cut, and the people came in, but only said to them: Take this girl away; I wish to sleep.

He slept and from that sleep, he has never awakened. But would not Holy Genghis heal himself? When he is healed, he will awake and save his people.

Source: Genghis Khan: Life, Death, and Resurrection by John Man