We talk about the great evils of the Warcraft universe - your Kil'jaeden's and Archimonde's, your Sargeras and your Old Gods - and of course there's the mortal monsters like Gul'dan, quite possibly the single most evil mortal being ever to live, or Kel'Thuzad who enslaved himself to evil so profound that his transformation into a lich was probably a palliative influence on his malevolence. But what of evil as a corrupting vector? What of an evil so profound that it is part of your blood, that can seep into the souls of an entire race? Ladies and gentlemen, I ask you - what of Mannoroth, king of the Pit Lords, master of the Annihilan?
We know little of the Annihilan. They are the pit lords, a massive and powerful species of demons recruited to serve the Legion by none other than Kil'jaeden himself. They were already a powerful part of the Legion when the War of the Ancients unfolded - Mannoroth himself was one of the Legion's elite who came through the portal to help make clear the way for Sargeras to stride forth into Azeroth, doing battle with the forces of the Kaldorei who were attempting to preserve their lives. But as to where the pit lords come from, it is as yet a mystery unsolved. What world did they originate on? What happened to it? We do not know.
What we do know is this - few races in all the cosmos can boast so powerful and corrupting an evil nature as the Annihilan. And Mannoroth was for eons first among the pit lords - the acknowledged master of his kind, the most powerful of them all. Not Brutallus, not Magtheridon, not Azgalor, but Mannoroth ruled the Annihilan on behalf of the Legion.
The War of the Ancients
As stated above, our first knowledge of Mannoroth comes from his appearance during the War of the Ancients. At the time, the forces of Queen Azshara, led by her chosen servant Xavius, were having limited success in battling those non-Highborne Kaldorei who were under the impression that the Highborne and their new demonic allies had taken Azshara prisoner. (It was unthinkable to the kaldorei of that time that their beloved Queen could possibly be working with the demons.) Sargeras was not pleased by the failure of Hakkar the Houndmaster to rout the disorganized night elf rabble. When Azshara suggested creating a shield spell that would seal the power of the Well of Eternity away from all in Azeroth except her and her Highborne, making it easier to devote all the Well's power to expanding the portal the Legion was using to invade it soon became apparent that none of the Highborne nor Hakkar were capable of creating the shield.
Thus came Mannoroth through the portal. Massive and terrifying, he quickly disposed of a Highborne who had the temerity to faint at the sight of him, but although rapacious and cruel he also proved to be a far more gifted sorcerer than any present save Azshara, designing the shield spell which cut the kaldorei (and indeed, all of Azeroth) off from the Well of Eternity. With all of the Well's magical power devoted to the portal, it quickly grew large enough to admit a terrifyingly vast demonic horde through, and Mannoroth then displayed that in addition to his sorcerous power, he was an extremely potent field commander.
In the end, however, despite Mannoroth's presence and a vast host of Legion demons, Azshara's Highborne failed and Sargeras was denied entry into Azeroth, although the cost was the sundering of the ancient continent of Kalimdor into four continents - Northrend, Pandaria, the Eastern Kingdoms and Kalimdor. Mannorth was banished back through the portal created above the Well of Eternity, his first failure in thousands of years of service to Archimonde and the Legion.
The Rise of the Orcish Horde and the Blood
That failure doesn't seem to have hurt Mannoroth's position among the Burning Legion - he remained Archimonde's second in command, head of the pit lords, and continued to spread destruction in the name of the Legion. But it certainly enraged the Annihilan - his long, long memory of the War of the Ancients and the beings who dared stand against the Legion would manifest in the future.
The role Mannoroth played in the corruption of the orcs is anything but subtle. Mannorth did not subtly work their beliefs and paranoia - that was Kil'jaeden. He certainly wasn't the one to willingly lead the orcs into their own damnation - that was Gul'dan. Mannoroth's role in their corruption was very, very simple. He was summoned onto Draenor by Kil'jaeden, and presented to Gul'dan. Gul'dan then called the orcs to the mountain known today on Outland as the Throne of Kil'jaeden. Once there, they were presented with a cauldron filled with the reeking, oozing, fel-corrupted blood of the pit lord, and upon drinking it they were infused with the blood lust of the Annihilan, transformed into a green skinned race of blood-crazed murderers, an army of berserkers and destroyers. First to drink from this cauldron was Grommash Hellscream. Thus, it was Grom who led his people into the slavery of the Blood Curse, bound to the Legion and Mannoroth for as long as the pit lord should exist. And since Mannoroth had already lived for at least ten thousand years at that point, it appeared that the slavery of the orcs was eternal.
Despite this 'blessing' the orcs ultimately failed the Legion. When Gul'dan opened a portal to Azeroth and the orcish Horde invaded, they had several early successes, but in the end were driven back and captured or destroyed. An effort by the fallen shaman Ner'zhul to use the skull of Gul'dan and other artifacts to create other portals like the one Gul'dan had made ripped Draenor apart, and as a result, the shattered land of Outland was created. The orcs had failed, and by extension, Mannoroth had failed again. The Annihilan now held Azeroth responsible for great humilation.
The Third War and the death of Mannoroth
The Third War, of course, was a full scale demonic invasion of Azeroth, essentially revenge for the defeat of the Legion at the Well of Eternity so long before during the War of the Ancients. But with the failure of the orcs to inflict their conquest on the world of Azeroth, the Legion turned to a new instrument, namely the Lich King and his Scourge. As a result, Mannoroth felt doubly humiliated, and as a result he was determined to reclaim the orcs that he thought of as his property due to the blood curse he'd inflicted upon them. Tichondrius, who was the handler for the Scourge, made sure to mock and torment Mannoroth whenever possible over the orcs failure. As a result, Mannoroth pursued them in a rage to Kalimdor, seeking to reclaim or destroy his errant property.
Upon arriving, however, it became clear that the night elves and the ancient Cenarius were still a formidable threat to the Legion's plan for Azeroth and the World Tree. Therefore, Tichondrius had a suggestion - the orcs, led by Hellscream, had angered the night elves and Cenarius by logging in their sacred forests, and would likely be destroyed by them. But if again corrupted by the blood of Mannoroth, they would become chaos orcs, infused with enough power to destroy the ancient and fully under the sway of the Legion once more. Mannoroth agreed, and spilled his corrupt burning blood into a nearby fountain of life. True to form, Hellscream found the fountain and drank from it. (Yes, it seems Hellscream's major problem was that he would just drink from anything.) And indeed, he was once again corrupted - after using the power granted by the blood to slay Cenarius, Hellscream was forced to submit by Mannoroth, who now controlled him utterly via the blood curse. Hellscream and those under his command were transformed into slaves by Mannoroth, and unleashed against their former allies in Thrall's Horde.
It's clear that Mannoroth savored the irony of this - Hellscream, the orc known as the Warlord of the Iron Will, was now his puppet, unable to resist his commands. However, Thrall managed to capture Hellscream (discovering the true origins of the Blood Curse in the process) and, alongside Cairne Bloodhoof and Jaina Proudmoore, freed him again from the Blood Curse's power. Mannoroth was displeased to lose his favorite 'pet' but he didn't give up on him - Hellscream was so amusing to the pit lord, and once fully under the influence of the Legion would serve as a fitting leader for a Legion-corrupted Horde. All that needed to be done was to remove Thrall from the equation.
Mannoroth chose to lure Thrall and Hellscream into the crater created by the infernals as they invaded Kalimdor, where he confronted the two orcs. The orcish people were doomed to be his slaves, he claimed, forever bound to him. After shrugging off Thrall's most powerful attack and crashing the warchief into a wall to lie unconscious and defeated, Mannoroth claimed that Hellscream could never be free of the blood curse. The boy believed you could be saved... but he didn't know, what burns within your soul. When in your heart, you know, we are the same! These turned out to be the last words Mannoroth ever spoke.
Perhaps he was right. Perhaps, in his heart, Grom knew that they were the same. But he didn't accept it - instead he chose to slay the Destructor, Mannorth king of the Annihilan, and die himself in the doing. A maddened charge, a savage wound to the pit lord's chest, and an explosion of fel power that claimed both their lives - thus did Mannoroth die, to the end convinced of his own superiority and the impossibility of overcoming the power of his blood.
The entire plot of Warlords of Draenor revolves around Garrosh Hellscream traveling to a Draenor where he was never born to prevent its Horde from becoming slaves to the Legion. He warns the Grommash Hellscream of this world not to drink the Blood of Mannoroth (which must have been hard, because as we've seen, Grommash will drink anything - obviously bubbling evil goop, random fountains in the middle of nowhere with witch doctors warning him not to take a drink, he don't care) and the cinematic we see above is the result - Mannoroth comes under concerted attack by the entirety of Draenor's Horde, and dies to an axe-blow to the skull after being struck in the chest by a siege weapon known as an iron star. This time, Garrosh manages to prevent Grom from being killed in the explosion, leaving Mannoroth dead... and his blood leaking from his corpse, and pooling into Draenor's soil.
What becomes of the body of this Mannoroth? What of his insidiously corrupting blood? Does the Iron Horde wisely destroy it, or do they make a mistake - do they seek to channel the power of it, make use of it, sure that this time they can avoid corruption? The future will reveal, but I'm not sanguine about it. The blood of Mannoroth is a potent evil, and no one who has ever sought to tap its power has come out of the experience whole.
While you don't need to have played the previous Warcraft games to enjoy World of Warcraft, a little history goes a long way toward making the game a lot more fun. Dig into even more of the lore and history behind the World of Warcraft in WoW Insider's Guide to Warcraft Lore.