Charles XII was the King of Sweden from 1697 to 1718 and by all accounts was pretty hardcore. He never registered physical or emotional pain, lived to drive his enemies before him, and was essentially an emotionless automaton who in the span of twenty-one years took the European country with the smallest population and briefly made it into one of the most formidable powers in the East, only to eventually watch everything crumble before him.
Charles assumed the throne of Sweden at age fifteen. Since Europeans were all pretty much dicks back then, a bunch of neighboring countries like Denmark, Russia and the Polish-Lithuanian Commonwealth all decided that they could beat the crap out of this kid and take his territories (back then, the Swedish Empire encompassed not only present-day Sweden but also the Baltic lands of Livonia and Estonia). Well Charles wasn't about to let that shit fly. He had made himself hardcore as a youth by spending his winters camped out in Swedish fir forests hunting wolves and bears just for the fuck of it, and he wasn't going to back down from a fight. Voltaire once quoted Charles as saying, "I have resolved never to start an unjust war but never to end a legitimate one except by defeating my enemies", which is pretty much old-school talk for, "I don't start shit. I end it."
So Charles raised an army and used his superior tactical skills to whomp some major Eastern European asses. First, he sent his men into Denmark where they put a stop to any plans the Danes had to occupy Sweden. Charles put his own man on the throne and ended that war pretty fucking quickly.
Well this totally pissed off Peter the Great, who sent Russian troops to occupy Livonia and Estonia and build a new city on Swedish turf. Charles was like, "oh hell no" and sent his troops to attack the main Russian army at the Battle of Narva in 1700. Charles and his force of about 10,000 attacked a Russian garrison of about 40,000 soldiers under the cover of a blizzard and despite being heavily outnumbered Charles won a sweeping victory, completely obliterating the Russian army while only losing about 600 of his own men.
After this, Charles turned his attention to the Polish-Lithuanian Commonwealth, who were formally declared as neutral but who were also definitely fucking with him. The Swedes beat the crap out of the Polish armies and Charles put another one of his cousins on the throne there. He became heralded as "The Alexander of the West" for his youth and military brilliance. An army directly under the control of Charles XII never lost a major engagement.
Well about this time Charles started to think he was pretty badass. Peter was remodeling his Russian army and provoking Swedish forces once again, and Charles decided it was time to end this shit. He made the same freaking mistake that so many conquerors would make after him and decided to invade Russia.
Charles, just like everyone else, got pretty much fucked in Russia. The weather was bad and the Russians were using "scorched earth" tactics to draw out the Swedish lines. Eventually, the Russians destroyed Charles' supply train and the cossack army that Charles' forces were counting on joining up with was completely annihilated by Peter's troops. Charles himself was wounded, and while he was in the hospital and unable to fight the Russians attacked his army at Poltava and dealt the Swedes a crippling blow. The Swedish army was crushed and Charles was forced to flee to the refuge of the Ottoman Empire.
In the Ottoman Empire, Charles kept trying to get the Sultan to piss off the Russians. Eventually the Sultan got sick of this crap and sent his men to kill Charles, but Chuck managed to escape and ride the entire width of Europe on horseback in about fifteen days. When he arrived back in Sweden he attempted to rebuild his army, but eventually got capped in the head by some idiot and died.
Charles was pretty badass. He completely abstained from alcohol and sex and was pretty much uncomfortable doing anything other than leading his troops to victory or being stoic as fuck. He lived fast, died young and when he went down he took the entire fucking country of Sweden with him. What more can you ask for from a historical badass?
You can read more about Charles XII of Sweden here.