It should be pointed out here that the timeline has seen some changes between the release of the original Warcraft game and World of Warcraft. I'm going with the most recent information I can, which means things like exactly when Blackhand became Warchief or how old Garona was may shift from what you remember from the RTS games. This is unavoidable, I'm afraid.
It is at this point that we must understand the nature of the two entities involved in this transaction, as it is their interaction here that fully changes the course of history for two worlds. Medivh was powerful, possibly more so than mortals can reckon even without the essence of Sargeras infesting him like a titanic parasite. He was intelligent, and tainted, seeing the very existence of the mortal races of Azeroth as an impediment to his quest for greater and greater understanding of magic. He believed he could manipulate Gul'dan into destroying the humans and other races of Azeroth by offering to the orc the location of the Tomb of Sargeras where his mother and predecessor Aegwynn had buried the corpse of the form Sargeras took to invade Azeroth a second time, in order to plant himself with Medivh (who was at this time unborn).
For all his power, and all his arrogance, the joined Medivh/Sargeras entity woefully underestimated Gul'dan, as had Kil'jaeden before him. Gul'dan wasn't insane. He wasn't misled. He wasn't misguided. Never for a moment did Gul'dan do anything without knowing exactly what he was about, what it would cost, and how it would benefit him. All Gul'dan cared about, from beginning to end, was Gul'dan: he only took actions that benefited others when it was unavoidably tied to his own welfare. Understanding that Kil'jaeden had abandoned him, Gul'dan agreed to Medivh-Sargeras' offer, but never intended to keep his end of it. While he didn't care if the Horde murdered every last living thing on Azeroth, he certainly didn't care if Medivh got what he wanted, either. Gul'dan wanted the tomb's location and that was all that mattered to him, the tomb, and the power he was sure it held, power to make him even greater than Kil'jaeden.
Each sure that he had hoodwinked the other, they began work on the Dark Portal between worlds. And in time, Medivh and Gul'dan succeeded (with the help of time travelers, wink win nod nod if you ran Black Morass) and the Horde entered into Azeroth. It should be noted that, although we today call them the Old Horde, they were simply The Horde then, a motley assortment of orc clans and even some ogres welded together by Warchief Blackhand the Destroyer, who believed himself Gul'dan's equal. It was Blackhand who served as titular head of the Horde and directed its military might against the humans of the Kingdom of Azeroth. At Blackhand's command (sorry, couldn't resist) they poured out of the Black Morass and locked into combat with the knights of the Brotherhood of the Horse.
A period of chaos and confusion followed as the Horde attempted to understand what had happened. Gul'dan had presented Azeroth as a ripe, rich world ready to be plucked, and all their observations had led the orc warriors to believe that humans couldn't possibly pose a threat to them. Having formerly been nomadic warriors and now addicted to the demon blood, run-ins with isolated human farmers gave them the idea that humans would be a far easier conquest than that draenei, who'd had access to strange magics after all. These fat sheep herders and cattle farmers were a threat to the Horde? Of course not! And indeed, isolated farms were no match for them.
But disciplined human military were. Humans had been fighting trolls for thousands of year at this point as well as fending off incursions by demons led to attack Azeroth by the existence of mages (this is why the Council of Tirisfal even existed, in point of fact) and had formed nations across the continent, Stormwind being only the closest to where the Dark Portal had been constructed.
The majority of the First War was fought under the aegis of Llane Wrynn, who was King of Stormwind at the time (he actually became King the same year as the orcish invasion, as his father King Adamant passed that year) and who was childhood friends not only with Medivh (the Guardian had grown up at Stormwind's court with his father Nielas Aran) but also Anduin Lothar, a member of the Brotherhood of the Horse who rose to prominence during the war and was also a direct descendant of the Arathor bloodline that had ruled the first human kingdom on Azeroth. Their initial victories left them unprepared for the true might of the Horde, however, as Gul'dan and Blackhand moved to quell the chaos and secure their leadership.
At this time Durotan and Draka, the leaders of the Frostwolves, found themselves exiled from the main body of the Horde due to their continual refusal to toe the Horde line under Blackhand. Seeking to prove that Gul'dan was working against the best interests of the orcish people, they contacted their old friend Orgrim Doomhammer, who agreed that something needed to be done about both his direct superior in the Blackrock clan and the shadowy Gul'dan. Unfortunately, not all of Doomhammer's chosen were truly loyal to him, and the Frostwolves lost both chieftain and chief's wife when Durotan and Draka were assassinated and their infant son left to die.
Meanwhile, events continued apace among the humans. Medivh fought a constant battle between the side of himself that was wholly in Sargeras' thrall and his own personality, and found himself saddled not only with an apprentice he didn't want (Khadgar, sent from Dalaran to learn from him) but also found himself in contact with Gul'dan through the spy and assassin Garona, of half orc/half draenei heritage. Hard as it is to believe, the sophisticated, mercurial, powerful Medivh was more attractive to Garona than the purely evil old orc warlock who'd force bred her by having orcs rape her draenei mother, then used demonic magics to age her to adulthood and used her as a spy and assassin while leaving her without real position or authority so that she was constantly imperiled by the blood addicted orc maniacs he forced her to interact with. It's hard to believe, really, that she might even consider disloyalty to a peach like Gul'dan, huh?
Eventually, despite their own differences, Khadgar and Garona became aware of Medivh's growing eccentricity. While they both felt loyal to the man, they investigated and, after experiencing strange visions and weird events in the tower of Karazhan, eventually determined that Medivh was in fact responsible for the Dark Portal. They turned to Medivh's oldest friends to help stop the wizard's plans, and it was Khadgar who helped Anduin Lothar kill his former master, enduring a curse that aged him into a withered old man in the process. This, however, did not save the Kingdom of Stormwind.
During the battle with Medivh, Garona had come into direct conflict with the mage. (Presumably this was well after he had, well, impregnated her.) He indicated disappointment at her choice to oppose him and during their conflict he cursed her to experience the division he himself had suffered while Sargeras had torn at his mind. This combined with the visions she'd seen while in the tower of Karazhan, and led her, while ostensibly loyal to King Llane and his trusted advisor on the Horde following Medivh's death, to eventually betray him. It was Garona who killed King Llane on Shadow Council orders while his son Varian witnessed the deed. This led to the fall of Stormwind.
Unfortunately for Garona, her 'master' Gul'dan had sensed Medivh's confusion when his former students and friends banded against him, and in a panic sought to ransack the Guardian's mind for the location of the Tomb of Sargeras. This meant that Gul'dan was in Medivh's mind when the wizard's body died and the forcibly joined souls of the human heir to Aegwynn and Aran and Sargeras were forcibly evicted from the body via swordpoint. This meant that Orgrim Doomhammer finally had the opportunity to avenge Durotan and Draka's deaths, since Gul'dan was nearly dead: he ripped off Blackhand's head and seized control of the Horde. Since Garona was a Shadow Council agent, Doomhammer held her in no specific regard, and had her tortured and questioned upon her return.
Meanwhile, as Doomhammer the Backstabber (as his own people chose to call him) was solidifying his position as new Warchief and rooting out and destroying Gul'dan's Shadow Council apprentices, Lothar led the retreat from Stormwind, as the Kingdom of Azeroth fell to the Horde. All was not yet lost for the humans, however, as several nations to the north were yet to learn of the true extent of the Horde's invasion, and Lothar intended to rally those people.
Next time on KYL, we look at the Second War, and discover the consequences. Why did we not see Grom Hellscream in the initial invasion when he was the first orc to drink demon blood? What role, if any, did Gul'dan play in the Second War? Why did the Horde seek allies among the trolls of Zul'Aman? All this and more next week.