Before there was Karl Franz, there was Magnus the Pious
Before there was Katarin, there was Miska the Slaughterer
Before there were autists in space and gay immortal pseudo-golems, there were plate-armoured Vikings
Before there were The End Times, there was the Great War Against Chaos
And before there was Archaon, there was Asavar.
We don't know where he came from (because GW never explains or go in depth around the origins of characters like this) but we do know that he was the chief of the Kul tribe and High Zar of the Kurgan, overall a pretty important position which led Asavar to being singled out by the Chaos Gods to become the 12th Everchosen. With his new job and the instructions to go bring ruin to the world, Asavar spent the next few years building up his forces into the largest Chaos invasion ever seen, and in 2301 IC he marched his troops South.
The Great War
Asavar moved into Kislev through a pass in the Northern Worlds Edge Mountains and, unlike the Germans and Napoleon, he did a good job at invading Russia, despite launching his invasion during the winter and ignoring what every /his/torian would be screaming at him. This is because of two glaring factors: firstly, the Warriors of Chaos are much tougher than normal men and live in a land even colder and inhospitable than Kislev, so the army would brush of blizzards like they were a spring breeze and secondly, Asavar's horde was so big that any casualties due to attrition were completely insignificant and had no effect on its fighting capacity, as discovered by the Ungol tribes who first saw the horde coming. The northern Ungol tribes act as both a warning system and delaying action for Kislev, as the tribes' mounted warriors usually spot approaching Chaos raids first, flank them, and start whittling them down with arrow fire, then when they get a decent estimate of the force's size they send some guys off to go inform the nearest settlements and cities in the horde's path so they can prepare. The problem the Ungols encountered when they ran into Asavar's army however, was that the fucking thing didn't end. Horsemen rode for hours and found no end to the horde, which was when they realised just how fucked everyone was. Most Ungol tribes rushed South to warn as many settlements as they could, with a handful staying behind to do what delaying skirmishes they could.
So informed about the incoming invasion, Kislev sent out a call for aid to its closest ally, the Empire, which unfortunately was on the final rounds of a 1000-years-long civil war, so not many Elector Counts were willing or able to send troops to assist. The one exception to this was Ostland, which sent an army marching North to reinforce the main Kislevite army. Unfortunately, even united these two armies were far too small to actually stop the Everchosen's legions, and during a battle at the River Lynsk the combined force was shattered. Asavar's army then marched on to Praag, where it met up with the force under the command one of his chosen lieutenants, Engra Deathsword, who had been laying siege to the city for something between 4 and 7 months despite his own large force. With all resources and defenders exhausted against the immense number of reinforcements, the city was finally overwhelmed in late 2302IC, the Chaos Warriors breaching the walls and slaughtering everyone inside. Asavar and Engra then took their combined force southwards towards Kislev city, aiming to inflict upon it the same fate before moving on to attack the Empire.
However by then the Empire had finally gotten their shit together, united under a noble from Nuln called Magnus, and was preparing two armies to head North aid the Kislevites, the first led by Magnus himself, and the second being composed of multiple Knightly Orders of the Empire from the Empire. The Dwarfs had also taken notice of the problems in Kislev and sent their own army to reinforce the capital directly, meaning when Asavar arrived at the city of Kislev he found not only a bunch of Slavs determined to put the defenders of Stalingrad to shame, but also a bunch of angry midgets who had built up the city's defences and dug in like ticks. The Everchosen launched his attack regardless (it was his job after all, and you try telling the Big 4 you're going home because you came across some stunties) and over the following weeks he would pull more and more Northmen and Chaos Warriors to him, starting to grind the defenders down. Just as the Chaos onslaught was about to shatter the Kislevite-Dwarvish army, the first army from the Empire arrived, and while they weren't expecting the hordes to already be besieging the city, they decided to go hell for leather into the back of the Chaos fuckers, forcing Asavar to turn part of his force around and lead a counter attack. The Warriors were able to push back and surround the Empire forces and stop a break out attempt by the Dwarfs, but then the the other Empire army, with reinforcements from the still-free parts of Kislev joined the fray, charging headlong into the flanks of the horde while the defenders sallied out once more, squeezing the core part of Asavar's forces between three prongs. The besiegers began to falter, and soon the Chaos Warriors started to disorderly try and push through in every direction.
It's unknown what happened to Asavar during the final battle of the war, as there are conflicting sources. The first claims that he and Magnus dueled, with Magnus managing to get the best of him when Sigmar himself imbued him with power. The second claims that the Chaos Gods abandoned him, taking his strength and leaving him to die, either as punishment for failing or for lols. And the third claims some upstart champion stabbed him in the back when he felt like the battle was lost, because you know, backstabbing to climb the ladder isn't just a Skaven thing. Whatever the truth, Asavar died on the field, and without him to hold the vast Chaos forces together the tribes scattered, either to return North immediately or raid smaller villages elsewhere. With the death of the twelfth Everchosen, the largest and most costly Chaos invasion ended, with the men of the North never posing such a dire threat since.
With the future (way in the future) release of the Warhammer: The Old World, there may be a chance that Asavar will be given a model and rules for the tabletop since TOW is set during the closing decades of The Age of Three Emperors, with his invasion being the catalyst which ended the period, meaning that it would make complete sense for him to be the big man of Chaos for the re-launch. GW themselves have also been hinting at this possibility as a recent FAQ on Warhammer community has suggested that TOW will go into more depth about Asavar's invasion. So who knows, maybe soon Archaon won't be the only Everchosen on the block.