Vlad the impaler, Fall of the Son of the Dragon
While Vlad was busy asserting power by UTTER BRUTALITY (seriously, that guy is like a mix between Settra and a Khorne Follower *I will put links on who they are), things outside of Wallachia have been moving. Returning to the Ottoman and Memeth, Vlad had initially promised to pay a yearly tribute not to have the Sultan's army steamroll him. Apparently, in the Wallachian's king language, an annual tribute means paying the first year and never sending anything again. Memeth, being frustrated by this, sent an envoy to see what the hell was going on. This is where the Ottoman sultan would find out how much of a nut job Vlad is. The envoy, barely arriving on Wallachian soil, got captured and was introduced to Vlad's favourite pass time via volunteering. (Read, they got impaled)
At war with the Ottoman
Immediately after this, Vlad sent his armies on a murder looting spree across the Ottoman territories, in the Balkan. Initially, during his assault, Memeth hadn't expected the Wallachian king to react this way so he wasn't really prepared to face the Son of the Dragon.
Despite the appearances, this attack actually was far from coming out of nowhere. You see, Memeth the Second was through and through a conqueror, and many (rightfully) expected him to invade Europe even more. The Papal State had even started to fund a crusade against the Ottoman after the fall of Constantinople, but things ran slow. War, despite popular belief, had little to do with religion. Sure, it was an excellent way to sway the mass into throwing themselves into meat-grinders, but it was often a power struggle and territory dispute for monarchs. And so, the crusade against the Ottoman was slowed down because Christian kings had as much, if not more, to fear of their fellow Christian neighbours. As such, in the greatest irony, the pope even asked other Muslims nations to attack the Ottomans!
Vlad also had his own personal reasons. Remember his brother Radu the Handsome? Well, he's still kicking around the Ottoman court and happens to be pretty close with Memeth. To Vlad, this was a clear threat to his throne, as the Ottomans could always replace him with his brother if they felt like it. Plus, his cousin in Moldavia, whom Vlad had helped retain his throne and served under his father (Vlad's uncle from the first episode), had joined the Ottomans. Politic is never personal. After all, Vlad sided with his father’s killer to win back his throne.
First Clash
Unfortunately for Vlad, and fortunately for everyone else, the free looting would not last as Memeth finally mustered his forces and met his old friend on the battlefield. And despite Vlad being a (bloody) badass, Memeth simply outnumbered him 6 to 1 and enjoyed having superior troop quality. Outmatched and outgunned, Vlad withdrew from the battlefield, leaving Memeth victorious. Knowing fully well that he couldn't defeat the might of the Ottoman head front, Vlad turned to a much different tactic.
Guerilla warfare
He ran away into the mountain, destroying and poisoning any supplies his troops couldn't carry. Then, they attacked the Ottoman’s troops with small lighting fast attacks to wither them down. The Wallachian king also used his experience serving in his foe's ranks years before in order to successfully send spies. Those spies proved themselves to be extremely successful soon enough. He learned of Memeth's locations in the camp, giving them a golden opportunity to strike a crippling blow.
Fall of the Son of the Dragon
One day, the Wallachian cavalry attacked the Ottoman camps by surprise, killing many before their enemies could even understand what was going on. Vlad himself stormed through the ranks straight toward the Sultan's tent, ready to kill his old friend. The attack was perfect, except for the little, tiny detail that Memeth was absent that day. Vlad was defeated, having lost his only chance of victory, and he fled to the mountains once again. Memeth quickly captured the Wallachian’s capital soon after but was met with all the Ottoman captives of the Wallachian, presented to him in the most Vlad way possible (Read, they got impaled).
And this is where Vlad's reign of terror kind of stabbed him in the back since everyone else happened to be sick of his antics. So, when Memeth arrived with a new contender to the throne, who wasn't some kind of murder maniac, let's just say that the boyars quickly chose their side.
Instead of being a tough warrior like Vlad, Radu was a skilled diplomate and was quick to convince everyone else that he was a better option than his psychotic brother. Meanwhile, Vlad, out of power and support, without an army left to fight, ran to the Hungarians for support. The latter happily replied by executing him. He wasn’t killed for his atrocities, but under the false pretext he was an Ottoman sympathizer. Just to show the Papal State they were ‘fighting’ in the after-mentioned crusade.
That was, kind of anti-climactic.