Unlike other KYL's we've written here, this one's actually about two things - a person, and a weapon. The weapon is one of the most powerful ever forged, crafted by the hand of a dwarven king mourning a brother he believed slain, made with a chunk of crystal from another world. The man was a tireless crusader for his people, who stood against the forces of the Scourge - and when he did, whole armies of the undead were as ash.
We speak of both, for both were the Ashbringer. But to speak of the blade, first we have to speak of the man - for it was by the actions of Alexandros Mograine that the blade came into existence.
We know that Mograine had two sons, Darion and Renault, and that he was married to Elenam who died after giving birth to his younger son Darion. Alexandros was one of the paladins who fought at the battle of Blackrock Spire. At this battle, while the orcs were being routed by Turalyon, Mograine encountered an orc warlock wielding a dread crystal that was suffused with such terrible fel energies that even after striking the warlock down, Alexandros was damaged merely by touching the item. This crystalline orb seemed to contain a shadow magic of unheard of power, and Mograine, despite having been maimed and withered in the hand that grasped it, believed there was merit in the item. So he chose to keep it hidden in a locked iron chest.
After the battle with the orcs, Alexandros Mograine retired to a rural life, doing his best to raise his sons alone in the absence of his deceased wife. And then the Scourge came to Lordaeron. Mograine knew he couldn't allow his native land to be destroyed by the foul forces of undeath any more than he could the orcs before them, and so he gathered those men and women of good conscience and strong faith in the light to meet him in Southshore, and presented them with the shadow orb he'd wrested from the orc warlock at Blackrock Spire.
Mograine proposed that if such an artifact of pure darkness could exist, so could an artifact that embodied and manifested the pure power of the Holy Light. Such an artifact, if it could but be found, could be the deciding factor that would prevent the Scourge from overwhelming their people. However, not all present at this meeting were as sanguine about the orb - after all, its shadowy power had blighted Mograine's hand, and they could all feel the pure darkness within it. An attempt to destroy the orb with the power of the Holy Light, however, had an unanticipated effect - instead of destroying it, the orb was transformed into the very artifact of the Holy Light Mograine had urged his fellow paladins to help him find. A pact was forged between them to find a way to use it to defeat the Scourge and rescue their people.
Mograine then traveled to Ironforge, to ask the King of the Dwarves Magni Bronzebeard for his aid in forging such a weapon. Magni had been informed that his brother Muradin was dead, slain by Arthas the self-same prince of Lordaeron who was now leading the Scourge against his own people. Enraged by the death of his brother (Magni had no way of knowing that Muradin wasn't dead, merely wounded and taken in by the Frost Dwarves) Magni set hammer to anvil and forged the blade Ashbringer, housing the crystal orb in the form of a radiant sun disk. Magni called the weapon his greatest creation, a weapon forged by his hand and suffused with all the grief he felt over his brother's death. Soon the weapon would be used to return that grief upon Lordaeron's undead assailants.
Mograine and his allies (Saiden Dathrohan, one of the first five paladins of Azeroth, among them) refused to submit to the Scourge and defied the traitor Prince Arthas' command to disband the Silver Hand. Unfortunately for the people of Lordaeron, they had more than just the Scourge to deal with - the demons of the Burning Legion were complicit in the Scourge's creation (it was Kil'jaeden who had created the Lich King, and three dreadlords who had been chosen to control him before Arthas engineered a rebellion after Archimonde's death and the Legion's defeat) and on one mission, Saiden Dathrohan died at the claws of Balnazzar the dreadlord. Worse for all concerned, however, was what happened after Dathrohan's death - Balnazzar chose to hide within the paladin's corpse, animating it with his own unholy essence and hiding within it.
By this time, Alexandros had become infamous as the Ashbringer, and the sword he wielding was named this in his honor. With this weapon he was effectively unbeatable. Whole Scourge armies were sent against him - he reduced them to ash. Realizing that Alexandros Mograine was a threat to him, Balnazzar made a deal with Kel'thuzad, lich servant of the Lich King and primary Scourge commander in what was now called the Plaguelands. Mograine would be destroyed. And proving once again that the Dreadlords are the ultimate in betrayal, Balnazzar chose to use Renault Mograine as the weapon to slay Alexandros.
Renault was jealous. Jealous of his younger brother Darion, jealous of his own father. The Dreadlord within Saiden Dathrohan found him a fertile ground, receptive to his machinations. Betray his father, and become a Highlord in the newborn Scarlet Crusade. A deal was struck. Balnazzar in the form of Dathrohan sent Alexandros and his old friend High Inquisitor Fairbanks to Stratholme, using Renault to trick the man. Renault abandoned his father to the tender mercies of a whole army of undead, but Alexandros destroyed wave after wave of them with the Ashbringer. In the end, it was not the Scourge that slew the Ashbringer. It was the Ashbringer that slew Mograine.
Exhausted after destroying countless waves of undead, Alexandros put his sword down to rest. In that moment, Renault returned and struck his father down, stabbing him through the heart with the Ashbringer itself. Just as the Holy Light had once redeemed the orb that became the weapon, this act of betrayal tainted the Ashbringer even as it linked the fate of Alexandros to that for the sword. Kel'thuzad, who had sent the army of undead, considered it a fair trade - now, the weapon that had once turned entire legions of undead to ash would be a servant of the Lich King, as would the blade he wielded.
Yet Alexandros, as Highlord of the death knights within Naxxramas and leader of the four horsemen, would not lie still in his service to Arthas. For his younger son Darion would lead an Argent Dawn force to besiege the floating necropolis and destroy Alexandros' undead form. This meant little, since Alexandros' spirit was bound to the blade, not his corpse. Darion retreated from Naxxramas with the blade, the sole survivor of the attack, and sought out his brother in the Scarlet Monastery, bearing the corrupted blade that had brought his father across into undeath.
Renault attacked Darion when he saw the blade, but it was too late for him. Alexandros' spirit lingered on in the sword. It rose, and Renault's pleas for mercy did not avail him - before Darion's horrified eyes, the father did to his son as his son had done unto him. Renault Mograine died at Alexandros' undead hands, impaled atop the Corrupted Ashbringer.
Darion's fate is known to us, of course. What is interesting to consider for now, however, is simple - where did that crystal orb come from? Why does it seem to absorb the Holy Light or the shadow, in turn? Is it related to the Naaru? Is the Ashbringer itself a Naaru trapped within a sword?
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.