Alp Arslan
"The Byzantine writers deplore the loss of an inestimable pearl: they forgot to mention that, in this fatal day, the Asiatic provinces of Rome were irretrievably sacrificed." - Edward Gibbon, Decline and Fall of the Roman Empire
Everybody knows the Turks were badass. That's a given. I mean, you can't read anything about the military history of Eastern Europe without being regaled with tales of insane, daring Christian knights demonstrating the quality of their solidified concrete holy nutsacks by standing up to the mighty, face-crushing Turkish hordes and somehow pulling victory from the jaws of inevitable defeat. So it stands to reason that since fighting the Turks and surviving without being impaled by your own spiked gauntlets was such a tremendous, noteworthy achievement, then these were obviously a ruthless, vicious, powerful civilization of earth-breaking killmongers so insanely tough that they ate baby reindeer and urinated Red Bull energy drinks, right? Well, if that's the case, why is it that most Westerners can't name a single notable Turkish hero aside from Suleiman the Magnificent, Barbarossa, and maybe Selim the Grim?
The Turks, much like the Persians, just need a better PR campaign or something, because they have a totally badass warrior pedigree on par with that of history's best. These guys forged a series of tempered-steel empires that dominated the fuck out of huge tracts of lands across Europe and Asia, and maintined a millenia-spanning stranglehold on Eastern military dominance running from the pre-Crusade middle ages to the end of World War I - a badass Xbox Live achievement that they owe in no small part to the warrior-king Alp Arslan and the role he played in the knife-point castration of the armies of Byzantium in the 11th century.
Adud Al-Dawla Abu Shuja Alp Arslan Muhammad succeeded his uncle Tughril Bey as Sultan of the Seljuk Turks in 1063. His first order of business was of course to crush all opposition to his accession and have any pretenders to the throne violently executed by the sharpest gardening instruments in the Turkish Empire. Once that was out of the way, Alp Arlsan decided to show the world that even though the Seljuks were newcomers to the scene in the Middle East, that this Sunni Muslim empire of nomadic Central Asian steppe warriors could get out there and brain fools with the best of them. Alp Arlsan, known as "The Valiant Lion" of the Turks, immediately assembled an appropriately-imposing force of Turkish cavalry and horse archers and set his sights on destroying all trace of the Byzantine Empire in Asia Minor.
At this point in history, the Eastern Roman Empire of Byzantium was still a pretty serious military threat, but years of über-corrupt rulers, reckless spending, and miscellaneous douchery left the entire system completely decrepit and over-the-hill, and Alp Arslan figured that he was just was the guy to deliver the final kick in the balls that was going to show the Greeks once and for all that they were no longer the big dogs in the Mediterranean. Alp and his armies figured the best way to demonstrate this fact was by blitzing into a number of small buffer states between Byzantium and the Seljuk Empire and utterly laying waste to everything they could find. The Turks, who were pretty pumped up about stabbing with intent to kill, found themselves facing off against Byzantine mercenaries who had gone months without getting paid and had shockingly little interest in getting themselves decapitated for a king who couldn't give a shit about them. The Turks captured Phyrgia, Cappadocia, Anatolia, and a bunch of other places that seem to only exist in ancient times, before facing stiffer resistance in Armenia and Georgia. The Armenians fought so hard and pissed Alp Arslan off so hard that after he finally captured their capital he ordered it torched to the ground and the entire population massacred. Hey, I didn't say he was a fine upstanding citizen – I said he was a dude you didn't want to fuck around with.
After pretty much smoking everything in Asia Minor by the end of 1068, Alp Arlsan decided to leave the mop-up work to his lieutenants, and he headed back home to his harem to chill with his plunder and babes and start planning an invasion of Fatimid Egypt (who, like Byzantium, was also pissing him off for some unspecified reason). Well no sooner did Double-A leave Asia Minor than some brave-yet-delusionally-misguided Greek Emperor decided he was going to put a humongoid army together and actually start fighting back against these unstoppable face-crushing marauders. Emperor Romanus IV Diogenes assembled a tremendous force of Macedonians, Greeks, Romans, Normans, Armenians, French, Modavians, and whatever else he could find (he even had a member of the Scottish royal family in his ranks) and set out to teach Alp a lesson in getting his face punctured by a flying sword-stroke of Western justice.
The balls-out assault caught Alp Arslan's men off-guard, and before you know it Diogenes started winning a bunch of victories and talking about how he was the fucking hottest shit since Greek Fire. The Turks were driven back beyond the Euphrates River by an army of 100,000 Western knights, and Diogenes laid siege to the remainder of the Turkish army at the city of Manzikert in 1071.