Ghengis Khan (in Mongolia, Chinggis Khaan), born Temujin , somewhere between 1155 and 1167, was the Mongol founder, or Khan, of the largest contiguous empire in history.  He formed the Mongol Empire chiefly by uniting the nomadic tribes of Northeast Asia and then following an aggressive foreign policy of invading East and Central Asia.  Ghengis Khan died in 1227. The death toll as a result of the Mongol conquests is estimated by Matthew White (in The Twenty (or so) Worst Things People Have Done to Each Other) as 40 million (White lists it as the third worst thing, just below WW 2 and Chairman Mao (there, death was mostly from starvation).  White's report is worth reading for any student of human behavior and history.   

The religion practiced by Ghengis was most likely Tengriism, although it is reported that he was interested in learning about other belief systems as well. Tengriism, a mix of shamanism, animism, and ancestor worship,  was the major Mongol belief system at the time, and the primary Tengrii deity  the Sky God.  The religion is still actively practiced in Mongolia (in parallel with Buddhism and Burkanism, a later belief system that encompasses codes of behavior, multiple deities, folksongs and  a recently recognized (but unsupported) relationship with Yoga and Buddhism). 

In recent times, the Ghengis Khan image has been revisited, and historians have pointed to the more  positive things he accomplished.  The Mongolian people, in general, believe him to be a Buddha, or at least the greatest  warrior hero in history.  In any case, the legends of this one man have brought a sense of national identity to Mongolians.

Geneticists have written about the tracing of the prolific and aggressive Khan Y chromosome,  some referring to his love life (with multiple consorts and 'others'), others concentrating on  his and his warrior's rapes and purposeful impregnation of conquered women. (See Skyes, Brian, Adam's Curse (Norton, 2004)