Sargeras is of course more powerful. Archimonde more directly destructive. But for sheer malice, for spite, for a turn of mind so devious and sinister that it pursues vengeance for over 25,000 years for the slight of not wanting to become a monster, you can never find evil more cunning and persistent than that of Kil'jaeden the Deceiver.
Make no mistake. Even Gul'dan pales in comparison to the hate, greed, and wanton cruelty that motivates Kil'jaeden. The demon lord par excellence, when we look at many of the greatest ills of the modern age it was the hand of the Deceiver that shaped them. The corruption of the orcs? The creation of the Scourge and the Lich King? The Third War? Others may have taken these actions, pursued them to their ultimate conclusion, but it was the mind of the Deceiver that brought them forth and worked to make them a reality. Indeed, it's fair to say that Kil'jaeden is often far more successful when he can resist taking an active hand in events - his most recent defeat at the Sunwell took place because he chose to attempt what Sargeras had failed to do and what had killed Archimonde, namely the bodily invasion of Azeroth proper.
So let us look now upon the Deceiver, lord of lies, spreader of falsehood - liar and betrayer of betrayers.
It's impossible to know if he understood what he was doing. Was he aware before he accepted that he was dooming his people to a demonic existence at the head of a force of cosmic evil that would sear entire worlds of life? Did he see the ultimate fate of existence at the hands of Sargeras, the destruction of world after world, countless lives hurled into the flames of oblivion? It's possible - even Velen admits that Kil'jaeden was among the cleverest of his people. Did Kil'jaeden deliberately, consciously throw his people to the flames for the sake of his own power? I wouldn't put it past him. One thing is certain, however - when his 'brother' Velen chose to flee rather than to embrace the same fate, Kil'jaeden took it more personally than even Sargeras did.
Sargeras appointed Archimonde as the head of his armies, the mailed fist that would crush worlds. Kil'jaeden was given a task suitable to his temperament and interests - he would be the general of spies, seducers and liars. It was Kil'jaeden's task to bring those that would submit under the banner of the Burning Legion, and he pursued his task with abandon. Many planets submitted to the Legion and their own destruction without a fight, softened up by the insidious invasion from within, betrayed by Warlocks who'd signed their own souls away for power. For it is the inevitable fate of all those who barter with the Deceiver to come away having lost everything. But Kil'jaeden didn't let this pursuit prevent him from seeking revenge - he pursued Velen and his 'draenei' across the cosmos. Whenever the exiles would find safe refuge, the forces of the Deceiver would be behind them.
This isn't to say that Kil'jaeden had nothing else to do but corrupt unsuspecting planets and chase after Velen. After Sargeras attempted a direct invasion into the world of Azeroth and was rebuked, Kil'jaeden was tasked with his greatest act of corruption yet. What was needed was a force that required no great expenditure of magical power to walk a mortal world, one that could simply walk on a planet's surface and lay waste to all it encountered. The Legion was rife with demons, but now Sargeras wanted a mortal army, and Kil'jaeden was the one to deliver it.
These two interests finally became one when Talgath, one of his best hunters, finally stumbled upon a world which proved to be very important to Kil'jaeden - a world that at first seemed merely one of many the draenei had briefly alighted on and then fled. But Talgath was astonished to find that this time, the draenei had not fled, but rather settled on this world, which they'd named Draenor, an ereduin word meaning Exile's Refuge. Talgath reported this back to Kil'jaeden, who was suitably pleased that his servant had finally located his 'brother' and the disloyal exiles. But what fascinated him even more were the natives of that distant world... the orcs.
These orcs were the product of a savage, hostile world and they'd grown strong in enduring and defeating the challenges of their world. Kil'jaeden saw the potential of that strength - with the right motivation, the orcs could be turned into a great engine of war. He began to consider a way that he could get the revenge he'd long been denied over 25,000 years of stalking his former friend and gain the army his master sought. And all it would take is a few lies.
Kil'jaeden, it must be pointed out, never forced the orcs to do anything. All he did was lie - all they had to do to not become genocidal murderers was to not believe him. But Kil'jaeden has had millennia of experience. He knows the lies people will want to believe. He first appeared to Ner'zhul, one of the oldest and most respected shamans on Draenor, in the form of his dead wife Rulkhan and he told Ner'zhul that the draenei were plotting against the orcs. Ner'zhul slowly came to the point of view Kil'jaeden wanted, and soon the Deceiver moved to appearing before Ner'zhul in his own form. This, however, ultimately proved to be an error on the Deceiver's part - Ner'zhul wondered why an angelic being and great ancestor spirit (which is how Kil'jaeden presented himself) would look so much like the very draenei he so loathed. Ner'zhul would in time go to Oshu'gun and learn that Rulkhan had never appeared to him, had never warned him of the draenei, and that the war against the draenei that he'd started was in no way approved of by the ancestor spirits. These spirits then turned their backs on Ner'zhul.
But was it an error at all? In one step, Kil'jaeden not only completely robbed the always doubting Ner'zhul of the shamanic power he would have needed to oppose Kil'jaeden's plan, he traded up in gaining the services of Gul'dan, Ner'zhul's former acolyte. Ner'zhul did Kil'jaeden's bidding because he'd been lied to, and once he discovered the truth he turned on the Deceiver. Gul'dan, though - in Gul'dan the Deceiver had found a servant willing to do anything for power. Through Gul'dan, Kil'jaeden finally destroyed the draenei - the orcs murdered the vast majority of the exiles, using their bodies to pave their roads in an act of ultimate defiling that even Archimonde couldn't have dreamed up.
Here's where things get interesting. Kil'jaeden went through all this trouble to create this army of corrupted, demon-blood addicted orcs, and as soon as he has them, he does nothing with them. Perhaps this is because Sargeras had plans for them - it was Sargeras who contacted Gul'dan from inside the body of Medivh and Sargeras who lead them to construct the Dark Portal and invade Azeroth. Kil'jaeden wasn't idle, however. After the mortals of Azeroth defeated the orcish slaves of the Legion and then invaded Draenor in turn, Ner'zhul used the power of various magical artifacts (including Gul'dan's own skull) to open portals and attempt to escape Draenor, tearing the planet apart in the process. But all he managed to do was deliver himself to Kil'jaeden.
The Deceiver could have killed his former servant. But that was neither cruel nor inventive enough for Kil'jaeden, and besides which the ancient Eredar Warlock is never wasteful nor quick to throw away good raw material. Instead, Kil'jaeden went as far beyond torture as a mind can possibly go - he very slowly tore Ner'zhul apart while confining his spirit to a magical suit of armor entombed in sorcerous ice, and he cast this frozen throne onto the glaciers of Northrend to become the Lich King. Thus, Kil'jaeden was not only responsible for the formation of the Old Horde, but for that of the Scourge as well (and thus, indirectly, to that of the Forsaken as well). Ultimately, the Scourge and the Lich King would play a powerful role in the destruction of Lordaeron and the summoning of Archimonde to Azeroth... and thus, to Archimonde's death.
While on the surface the betrayal of the Lich King and the death of Archimonde is a setback for the Legion, it certainly worked out for Kil'jaeden. With Sargeras trapped in the Twisted Nether and Archimonde destroyed, the control of the entire Burning Legion falls to Kil'jaeden, who is now the acting supreme commander. One wonders if the Lich King was always intended to betray his masters. Kil'jaeden next entrusted Illidan with the task of destroying the Lich King in exchange for an offer of power, yet when Illidan failed and Arthas Menethil became one with the Lich King, Kil'jaeden didn't really move against Illidan. He made threats, and sent loyal Legion forces to do battle with those demons under Illidan's control - but Kil'jaeden had displayed the ability to move directly against Outland if he so chose, since it was a world suspended in the Nether itself. Why hasn't he?
Indeed, Kil'jaeden's most recent moves - recruiting Kael'thas Sunstrider, attempting to bodily move through the Sunwell and into Azeroth proper - don't seem much like him at all. One has to wonder why he would suddenly decide to make so overt an attack. The idea of overweening pride seems possible. He states as he stepped through the Sunwell that he intends to do what Sargeras could not. But it seems frankly a little too repetitive and unoriginal for the Deceiver, who came up with the Lich King after all. But then again, he is called that, isn't he? Kil'jaeden the Deceiver, the ultimate liar. And this makes us wonder if perhaps everything he's been doing - recruiting Illidan, allowing the Lich King to run wild, forcing us to unite against him at the Sunwell - is part of some greater feint, some larger piece of misdirection. Give us enemies to fight, keep us busy, and make us believe the Legion has been vanquished at minimal cost to him.
One thing is clear. The Deceiver yet awaits us. The Legion is coming. And we may be deceived by it sooner, rather than later.
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.