The man who would become the “Great Khan” of the Mongols was born along the banks of the Onon River sometime around 1162 and originally named Temujin, which means “of iron” or “blacksmith.” He didn’t get the honorific name “Genghis Kahn” until 1206 when he was proclaimed leader of the Mongols at a tribal meeting known as a “kurultai.” While “Khan” is a traditional title meaning “leader” or “ruler,” historians are still unsure of the origins of “Genghis.” It may have meant “ocean” or “just,” but in context, it is usually translated as “supreme ruler” or “universal ruler.”

From an early age, Genghis was forced to contend with the brutality of life on the Mongolian Steppe. Rival Tatars poisoned his father when he was only nine, and his own tribe later expelled his family and left his mother to raise her seven children alone. Genghis grew up hunting and foraging to survive, and as an adolescent, he may have even murdered his own half-brother in a dispute over food. During his teenage years, rival clans abducted both he and his young wife, and Genghis spent time as a slave before making a daring escape. Despite all these hardships, by his early 20s, he had established himself as a formidable warrior and leader. After amassing an army of supporters, he began forging alliances with the heads of important tribes. By 1206, he had successfully consolidated the steppe confederations under his banner and began to turn his attention to outside conquest.

For an influential figure, very little is known about Genghis Kahn’s personal life or even his physical appearance. No contemporary portraits or sculptures of him have survived, and what little information historians do have is often contradictory or unreliable. Most accounts describe him as tall and strong with a flowing mane of hair and a long, bushy beard. Perhaps the most surprising description comes courtesy of the 14th-century Persian chronicler Rashid al-Din, who claimed Genghis had red hair and green eyes. Al-Din’s account is questionable—he never met the Khan in person—but these striking features were not unheard of among the ethnically diverse Mongols.

The Great Khan had a keen eye for talent, and he usually promoted his officers on skill and experience rather than class, ancestry or even past allegiances. One famous example of this belief in meritocracy came during a 1201 battle against the rival Taijut tribe when Genghis was nearly killed after his horse was shot out from under him with an arrow. When he later addressed the Taijut prisoners and demanded to know who was responsible, one soldier bravely stood up and admitted to being the shooter. Stirred by the archer’s boldness, Genghis made him an officer in his army and later nicknamed him “Jebe,” or “arrow,” in honor of their first meeting on the battlefield. Along with the famed general Subutai, Jebe would go on to become one of the Mongols’ greatest field commanders during their conquests in Asia and Europe.

Genghis Khan often gave other kingdoms a chance to peacefully submit to Mongol rule, but he didn’t hesitate to bring down the sword on any society that resisted. One of his most famous campaigns of revenge came in 1219 after the Shah of the Khwarezmid Empire broke a treaty with the Mongols. Genghis had offered the Shah a valuable trade agreement to exchange goods along the Silk Road, but when his first emissaries were murdered, the enraged Khan responded by unleashing the full force of his Mongol hordes on the Khwarezmid territories in Persia. The subsequent war left millions dead and the Shah’s empire in utter ruin, but the Khan didn’t stop there. He followed up on his victory by returning east and waging war on the Tanguts of Xi Xia, a group of Mongol subjects who had refused his order to provide troops for his invasion of Khwarizm. After routing the Tangut forces and sacking their capital, the Great Khan ordered the execution of the entire Tangut royal family as punishment for their defiance.

While it’s impossible to know for sure how many people perished during the Mongol conquests, many historians put the number at somewhere around 40 million. Censuses from the Middle Ages show that the population of China plummeted by tens of millions during Khan’s lifetime, and scholars estimate that he may have killed a full three-fourths of modern-day Iran’s population during his war with the Khwarezmid Empire. All told, the Mongols’ attacks may have reduced the entire world population by as much as 11 percent.

Unlike many empire builders, Genghis Khan embraced the diversity of his newly conquered territories. He passed laws declaring religious freedom for all and even granted tax exemptions to places of worship. This tolerance had a political side—the Khan knew that happy subjects were less likely to rebel—but the Mongols also had an exceptionally liberal attitude towards religion. While Genghis and many others subscribed to a shamanistic belief system that revered the spirits of the sky, winds and mountains, the Steppe peoples were a diverse bunch that included Nestorian Christians, Buddhists, Muslims and other animistic traditions. The Great Khan also had a personal interest in spirituality. He was known to pray in his tent for multiple days before important campaigns, and he often met with different religious leaders to discuss the details of their faiths. In his old age, he even summoned the Taoist leader Qiu Chuji to his camp, and the pair supposedly had long conversations on immortality and philosophy.

Along with the bow and the horse, the Mongols most potent weapon may have been their vast communication network. One of his earliest decrees as Khan involved the formation of a mounted courier service known as the “Yam.” This medieval express consisted of a well-organized series of post houses and way stations strung out across the whole of the Empire. By stopping to rest or take on a fresh mount every few miles, official riders could often travel as far as 200 miles a day. The system allowed goods and information to travel with unprecedented speed, but it also acted as the eyes and ears of the Khan. Thanks to the Yam, he could easily keep abreast of military and political developments and maintain contact with his extensive network of spies and scouts. The Yam also helped protect foreign dignitaries and merchants during their travels. In later years, the service was famously used by the likes of Marco Polo and John of Plano Carpini.

Of all the enigmas surrounding Khan’s life, perhaps the most famous concerns how it ended. The traditional narrative says he died in 1227 from injuries sustained in a fall from a horse, but other sources list everything from malaria to an arrow wound in the knee. One of the more questionable accounts even claims he was murdered while trying to force himself on a Chinese princess. However he died, the Khan took great pains to keep his final resting place a secret. According to legend, his funeral procession slaughtered everyone they came in contact with during their journey and then repeatedly rode horses over his grave to help conceal it. The tomb is most likely on or around a Mongolian mountain called Burkhan Khaldun, but to this day its precise location is unknown.

Genghis Khan is now seen as a national hero and founding father of Mongolia, but during the era of Soviet rule in the 20th century, the mere mention of his name was banned. Hoping to stamp out all traces of Mongolian nationalism, the Soviets tried to suppress the Khan’s memory by removing his story from school textbooks and forbidding people from making pilgrimages to his birthplace in Khentii. Genghis Khan was eventually restored to Mongolian history after the country won independence in the early 1990s, and he’s since become a recurring motif in art and popular culture. The Great Khan lends his name to the nation’s main airport in the city of Ulan Bator, and his portrait even appears on Mongolian currency.