Genghis motherfucking Khan
Despite being very similar, he is not the same person as Jaghatai Khan.
"Oh people, know that you have committed great sins, and that the great ones among you have committed these sins. If you ask me what proof I have for these words, I say it is because I am the punishment of God. If you had not committed great sins, God would not have sent a punishment like me upon you."
- – The man himself
Chinggis Haan (more commonly known in the English-speaking world as Genghis Khan and Genghis 'Motherfucking' Khan to the assholes of /tg/) was a 12th century Mongolian warlord (Born AD 1162 Died AD 1227) and Khagaan (Emperor) who, in AD 1206, succeeded in uniting the Mongol tribes of Northeastern Asia into what would become known as the Mongol Empire. Contrary to popular belief, he was not the first individual chieftain to attempt in creating a unified Mongol confederation; the first having been his great-great grandfather Habul Haan who established the Khamaag Mongol confederation. Chinggis was infinitely more successful though, and managed to unite nearly all the disparate Mongol factions into his empire; which is why he is considered by modern day Mongolians as the founding father of the Mongol state.
Born Temujin, meaning 'man of iron' big daddy Genghis' and thought to be derived from a Northeastern Turkic word, he was the son of Yesukhei (later to be known as Yesukhei Bahadur or 'Yesukhei the Warrior' after being dubbed posthumously so by Chinggis), who was chieftain of the Borjigid tribe (Grey-Blue wolf tribe) and descended from the legendary warrior-king Bodonchar Munkhaag (lit. Bodonchar the Bastard, dubbed so for his illegitimate birth). In turn, Temujin was also descended from Borte Chino, the great wolf and mythical forebear of the Borjigid tribe, as well as from Tengri, the regional equivalent of Tyr.
Chinggis is notable primarily for being a warlord and having waged the Mongol Invasions of Asia and Europe, which collectively resulted in the massacre of 40 million people; which amounts to 10% of the world's population at that time. He's also notable for having 0.4% percent of the world's population springing forth from his mighty loins, which makes him akin to a real world equivalent to Walder Frey from A Song of Ice and Fire. Or at least he would if Frey was an apocalyptic level badass and not an underachieving old lecherous fuckwit. For obvious reasons, most of these people tend to be from Central Asia (Mongolia, Kazakhstan, Kyrgyzstan, Uzbekistan, Turkmenistan, East Turkestan, Tajikistan, Afghanistan), some parts of South Asia (primarily northern India and Pakistan) and Siberia and Turkic Russia (Republic of Tatarstan, Republic of Baskhortostan) and most of them are Muslim. Descent from Chinggis Haan is so wide-spread that it even has a name; Altan Urag, which translates roughly to 'Golden Lineage' and is also the name of that Mongolian folk-rock band. In Kazakh, this is called 'Töre'. Historically, a person descended from Chinggis Haan would be called 'Chinggisid', in the tradition of Central Asian tribes deriving their names from their founders and adding '-id'; meaning 'descended from', compare with the Scandinavian '-inga' suffix.
Despite his highly noble birth, Chinggis was born at a time when mediaeval Mongol tribes were constantly at each other's throats in a manner not dissimilar to how the Warriors of Chaos in Warhammer Fantasy are. Every fucking second of every fucking day until the Everchosen decides to smack heads together. Incidentally, the Kurgan, one of the three groups comprising the Warriors of Chaos, is based off the Mongols. Except Mongolia is less cold and marginally less dangerous than Norsca. Chinggis would grow up to be the Everchosen of this scenario, in terms how apocalyptic his eventual rise to power would be to the people south of him, much like how the Everchosen is. Hell, he even claimed divine favour from Tengger Etseg (lit. Sky-Father) - the Mongol god of thunder and head of the Mongolian pantheon. For much of his childhood however, his lot was to be married off by his father Yesukhei in order to secure an alliance with the Khongirad tribe; where he would later meet his wife Borte. Unfortunately for the future Chinggis Haan, daddy ran afoul of the neighbouring Siberian Tatar tribes to the North and ended up with his skull being used for some Tatar Haan's koumyss. After that, Temujin's family was swiftly usurped from the position of chieftain by one of his father's anda (bondsman, essentially) and so was he cast out with his brothers Begter, Hasar, Hachiun and Temuge and his mother.
Despite only being 12, Temujin managed to look after his family, making ends meet and carving out an existence in one of the most hostile environments on earth. He even killed his elder brother Begter over a dispute over hunting spoils, to give you an idea of how harsh living conditions were at the time. Interestingly, around this time he was captured and made a slave by the Tayichi'ud tribe of Mongolia, but managed to escape through his balls-to-the-wall insanity and the aid of a sympathetic guard. He apparently must have pulled some Conan the Barbarian level shit there, since he earned something of a mythical reputation shortly thereafter. Later on, he reunited with Borte and married her, only to have her stolen by the Merkit tribe (bride-kidnapping was nothing unusual in Mongolia at the time and is still used symbolically in some Central Asian countries as a sort of courtship ritual). Despite the fact that most Mongols at the time would simply shrug and steal another wife from somewhere else, Chinggis was not one to accept defeat willingly and swiftly gathered warriors to raid the Merkit tribe in order to recover Borte. A particularly bloody affair it would seem, as a few folk-tales supposedly tell of how Chinggis devoured the perpetrators of the kidnapping in a shamanic ritual. What is confirmed however is that whatever Chinggis and his raiders did must have absolutely slaughtered the Merkits, since by the time he was declared Khagaan shortly afterwards they ceased to exist as a distinct tribal group, with the survivors being absorbed by the other tribes or fleeing northwest to be subsumed by the Qipchak Turks. This and other references to his brutality is what leads many elegen/tg/entlemen to suspect he was a champion of Khorne.
Incidentally, Turko-Mongolian paganism really does have a red war-god associated with battle-frenzy; Kyzaghan, who may or may not actually have been one of Khorne's many guises during medieval Terran days, just like with Odin.
After defeating all his tribal rivals and becoming Khagaan of the Mongol Steppe, Chinggis turned his wrathful blue-eyed gaze (yes, really! http://www.history.com/news/history-lists/10-things-you-may-not-know-about-genghis-khan) south to the Jin empire, which had long played the Northern tribes off against each other in order to protect their northern borders. Not one to allow the sallow skinned southern pansies to get the better of the stronk manly northerners, Chinggis marshalled his forces to end the Jurchen domination over Central Asia.
Like Angron's Dominion of Fire, Chinggis Haan's invasion of the southern Jurchen empire was a tremendous success, his hardened Mongol tribesmen slaughtering the weak, soft-bellied Jin armies like wolves among sheep (an actual expression in Turko-Mongol battle poems). In time, he succeeded in extracting vassalage from the northerly Chinese empires. After the resounding success he had garnered against the Jin and Tangut Xi-Xia, he turned his attentions to the Kara-Khitai Haanate which was under the command of the deposed Mongol chieftain Kuchlug, who had previously headed the Naiman clan which Chinggis had defeated and pushed out of Mongolia in his quest to unite the clans. The Mongol invasion of Kara-Khitai saw its borders extent to Lake Balkhash in present day Kazakhstan, which later brought it into conflict with the Khwarezmian Empire. Though Chinggis died in 1227, his son Ögedei Khan and grandsons Kublai Khan and Möngke Khan would expand the empire into its peak, conquering all of China and pushing into Iraq until he was stopped by the Mamluks in Egypt. Unsurprisingly, the Mamluks themselves were Qipchak warriors closely related to the Mongols by blood and lifestyle. In the end, the only thing that could truly halt the Steppe Warriors was other Steppe Warriors (and Vietnamese).
End of an Empire
After years of civil war and succession issues, the empire was divided into four giant segments in the late 13th century between his four sons Ögedei, Tolui, Jochu, and Chagatai. These separate Khanates would later fragment and dissolve by the 16th and 17th centuries, the great Mongol Empire of Chinggis never to return. The closest the successors of Chinggis ever came to recreating his empire was his great-great-grandson Timur (Tamerlame) in the late 14th century, who ran around central Asia kicking ass left and right and not giving a single damn who stood in his way. Genghis Khan's descendants and succesor kingdoms tended to pop up again and again throughout history; with Hulagu Khan laying siege to and utterly razing Baghdad in 1258, Yonten Gyatso (a descendant of Kublai) becoming the 4th Dalai Lama and Ablai Khan in the late 18th century uniting much of Central Asia into what is now modern-day Kazakhstan. For centuries after his conquest, royalty from Mughal Emperors to Czars of Russia could possibly relate to Chinggis in some way.
In the 40k Fandom
800 years in the future, mankind is so impressed by Chinggis that he is inserted as an honorary warrior in everybody's favorite grimdark wargames series. And what faction could possibly be worthy of this riding, bearded, Mongolian, head-chopping conqueror with "Khan" in his name? That's right, the Daemons of Khorne. It's long been accepted in the fandom that upon the "death" of Chinngis he actually achieved apotheosis and became Doombreed, the first daemon prince who was explicitly noted to have been offered daemonhood before the Emperor revealed himself to humanity.
The GW writers saw our devotion and decided to sorta kinda canonize this theory with another nicely copyrighted name they can sell us models of. In Imperial Armor Volume Seven, Siege of Vraks part three, Uraka Az'baramael, the Warbringer, strides into the lore. He's explicitly stated to have lead "horse-bound armies across Ancient Terra" and that almost exactly ten years before his death, "Upon taking one city by siege, he ordered the entire populous massacred and thousands were beheaded," an act that so pleased the Blood God he was offered a chance at daemonhood. All he had to do was to behead a Bloodthirster, but that's no hard feat for Chinngis. He's been terrorizing Grey Knights ever since.