A series of five books by Conn Iggulden which tell the story of the Mongol Empire. They are:
Wolf of the Plains (2007) (Published in America as Genghis: Birth of an Empire): Follows Temujin, son of Yesugei, as he is banished from his tribe and goes on to not only survive, but begin to unite all the people of Mongolia, becoming Genghis Khan.
Lords of the Bow (2008): Having crushed the Tartars and united the Mongols into a single nation, Genghis Khan turns his attention to the tribes' traditional oppressors, the Xi Xia and Chin empires in what is now northern China.
Bones of the Hills (2008): Xi Xia and Chin are under Mongol domination, but Genghis Khan's ambassadors to Khwarezm are tortured and killed. The Mongols move against the Arabs in revenge, and their armies reach as far west as Russia.
Empire of Silver (2011) (Published in America as Ogedai: Empire of Silver): Ogedai, Genghis' grandson, hasn't assumed the title of Great Khan even though his claim is the strongest; he also suffers from a weak heart and his brother, Chagatai, intends to wrestle the rulership from him. Tsubodai, Genghis' old friend and comrade, will also lead the Mongol armies ever further into the West, fighting the Russian Empire and the Templar Knights.
Conqueror (2011): The last book in the series, it follows the reigns of Guyuk Khan, son of Ogedai Khan, and Mongke Khan, Guyuk's cousin, and the military deeds of Kublai, brother to Mongke Khan, against the Sung Empire (southern China, nowadays) as he is forced to change from a scholar to a warrior and leader of men.
Badass: Temujin, Yesugei, Arslan, Jelme... Pretty much everyone who isn't Temuge. But Temujin stands out, being able to beat the First Sword of Kaifeng in a fight in heavy armour he's unaccustomed to, throttle men easily, lift a man above his head and break his back over his knee, and able to fight one of his best soldiers to a stand still in old age.
Badass Grandpa: Arslan. Temujin too in the later chapters of Bones of the Hills.
Badass Army: The Mongol army under Temujin, the Jihadis under Jelaudin.
Badass In Charge: Temujin, first of raiders in the North, then of the Wolves, then of Mongolia, and then of Chin, Koryo and Khwarezm.
Four-Star Badass: Temujin, Tsubodai, Jochi, Jelaudin... the books are saturated with them.
Blood Knight: Temujin, it's easy to see why the Warhammer fandom thinks he worships Khorne.
We can live our lives at peace, so that our sons and grandsons ca live their lives at peace, but what is the point? If we all live to eighty in a green field without ever holding a bow or sword, we will have wasted the good years. You should know the truth of that. Will our grandsons thank us for a peaceful life? Only if they are too afraid to take up arms. I would not wish a quiet life on my enemies, Kachiun, never mind my own family. Even cities only prosper when there are rough men on the walls, ready to fight and die so that others may live. With us, we all fight, from the first yell to the last breath. It is the only way to take pride in who we are.
Grim Up North: The Tartars seem to thrive in the frozen wastes of Siberia. The Mongols themselves to the Chin and Khwarezm. They constantly complain of the hot weather of these lands and how they prefer their own icy homeland, and how the winters there are little more than spring compared to what they go through. They're like the Vikings of the east.
Grey and Grey Morality: In the first novel Temujin comes across as a straight up hero by the standards of his time, fighting to keep his family safe and evading the wrath of the traitorous and cruel Eeluk. However, he's also the brutal leader of savage northern raiders as he attacks the Tartars, killing their men and raping their women to get back at them for killing his father. In the later chapters of Wolves of the Plains, Temujin's sheer wrathful nature and ambition also begin to surface. By the time Bones of the Hills comes out, he basically goes to Khwarezm to avenge the torture and murder of his emissaries at the hands of Inalchuk, but he also makes it a point to wipe out his entire city and people to drive the point home that he's not to be fucked with, slaughtering millions (and thus, it's implied, indirectly causing the virulent plagues that sweep Khwarezm). Meanwhile, the Shah, while mostly a good man, becomes consumed with hatred by seeing what Genghis Khan does to his people and attempts to launch a lightning raid against the undefended Mongol women and children. The closest thing Bones of the Hills have to a good guy is Jelaudin, because he's basically fighting to free his country from Mongolian occupation and avenge their atrocities. And even then, the Mongols really aren't that oppressive towards the people of Khwarezm.
There's a deliberate contrast set by the author between Hulegu and Kublai's methods of conquest in Conqueror.
Historical Villain Upgrade: The Tartars are responsible for a fair bit more of the crap in Temujin's early life than happened in reality. For example, they're written as the ones who raped Borte; in truth, that was done by the Mergid people.
Munafiqeen: Inalchuck. By day he fasts in Ramazan, prays diligently, gives charity and reads the Holy Qur'an. By night, he drinks heavily, engages in deprave sex acts with his slave girls, and tortures Mongolian diplomats.
Horse Archer: The Mongols. The Turks in Jelaudin's army are also this, and just as good. Given that Turks and Mongols are closely related and share a culture.
Shut Up, Hannibal!: Genghis and Tsubodai do this to the Old Man of the Mountains. After he accuses Genghis of being an idiotic destroyer who can accomplish nothing of his life. I guess forging an entire nation out of barbarian clans and giving it order and purpose doesn't count.
I needed to laugh. I needed to stride halls filled with dead whores and have an assassin tell me I've made nothing in my life.