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 murder Vikings
Before there was The End Times, there was 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 the High Zar of the Kurgan. Being king shit of chaos, led Asavar to being singled out by the Chaos Gods to becoming the 4th 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 force ever seen, until 2301 IC he launched his invasion of the south.
The Great War
Asavar marched his force into Kislev through a pass in the Northern Worlds Edge Mountains and unlike the Germans and Napoleon, Asavar 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. 1; 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 2; Asavars horde was so big that any casualties due to attrition were completely insignificant and had no effect on the hordes fighting capacity. This was discovered by the Ungol tribes who first saw the horde coming. The northern Ungol tribes act as both a warning system and delaying action, as the tribe will get around to the sides of a horde and start picking a peeling off parts of the chaos forces with mounted arrow fire, and when they get a decent estimate of the forces size would send some guys off to go inform the nearest settlements and cities in the hordes path so they could prepare. The problem that the Ungols encountered when they ran into Asavars horde however, was that the fucking thing didn't end. Horse men would ride for hours and find no break in the horde, which was when they realised just how fucked they were and most Ungol tribes rushed south to warn as many as they could, with a handful staying behind to do what delaying skirmishes they could.
Hearing about the incoming Horde, Kislev sent out a call for aid though with it's closest ally, the Empire, being pre-occupied having civil war that had lasted just under 1000 years and so not many Elector Counts were willing to send troops to Kislev with the exception of Ostland who sent an army marching north to reinforce the main Kislevite army. Unfortunately these two armies were far too small to actually stop Asavars horde and during a battle at the River Lynsk the defending force was shattered. Asavars force then met up with the force under the command one of his chosen lieutenants Engra Deathsword outside of Praag who had been laying siege to the city. Despite having such a massive army attacking the city, Praag had held for quite a while (probably between 4-7 months) but when Asavars army linked up with Engras the city was finally overwhelmed in late 2302 IC after all the resources and defenders had exhausted themselves and the Chaos Horde shattered their way inside and slaughtered everyone inside. Asavar and Engra then took their combined force southwards towards Kislev (the city) to destroy it and they would make tracks into the Empire.
However within the time Asavar had launched his invasion, the Empire had finally got their shit together after their war and united under a noble called Magnus from Nuln and was preparing two armies to head north aid the Kievites, the first was to be led by magnus and the second was to be made up of multiple Knighty Orders from the Empire. The Dwarfs had also noticed the problems in Kislev and had sent their own army north to reinforce the capital directly, meaning when Asavar arrived at the city of Kislev he found not just 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 cities defences and dug in like ticks. Regardless Asavar launched his attack of the capital and over the next weeks would pull more and more northmen and Chaos Warriors to him and started grind down the defenders and just as the forces of the Chaos onslaught was about to shatter the defences, the cliched miracle occurred. The first Empire army lead by Magnus arrived at Kislev, and while they weren't expecting the Chaos hordes to already be at the city they decided to go hell for leather into the back of the Chaos horde and Asavar had to turn around part of his force and lead a counter attack to push back the Empire, which was able to push back and surround the Empire forces and stop a break out attempt by the Dwarfs. And then the second cliched miracle occurred as the second Empire army, with Kislevite reinforcements arrived and also charged headlong into the chaos horde while the defenders made a second breakout attempt causing the core part of Asavars army to be attacked on three fronts, causing it to finally began to falter and soon the Chaos Warriors started to try and make their own break out attempts, with warriors killing and rushing in every direction.
It's unknown what happened to Asavar during the final Battle of the war as their are conflicting sources. The first claim that he and Magnus had a duel, with Magnus managing to get the best of him when Sigmar himself imbued him with power. The second claim that the Chaos Gods abandoned him, taking his strength and leaving him to die, either as punishment for failing or just for lols. And the final claim is that some upstart champion stabbed him in the back when he felt like the battle was lost, to further rise up the ranks and gain favour with the gods. The truth is that regardless of the cause, Asavar died on that field during the battle and without him to hold the chaos forces together they scattered, either to return north or raid smaller villages in their own smaller warband. And with the death of the fourth 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 table top since TOW is set during the closing decades of The Age of Three Emperors and Asavars invasion was the catalyst which ended the period, meaning that it would make complete sense for him to be made the big man of Chaos for the game. GW themselves have also been hinting at this possibility as recent FAQ on Warhammer community has suggested that TOW will go into more depth about Asavars invasion. So who knows, maybe soon Archaon won't be the only Everchosen on the block.