"In the north of the world the forces of Chaos gather, awaiting their moment to strike. At their head is the Everchosen, the warrior who will lead the final, cataclysmic assault that will usher in the End Times and the reign of the Ruinous Powers. But he was not always thus – he was once a man, a devout servant of the warrior-god Sigmar. What could cause such a soul to fall to the worship of the Dark Gods? What dark events could have put a knight of the Empire on the path to becoming the harbinger of the world’s end? And just who was the man who will become known to all as Archaon?" -Archaon - Everchosen by Rob Sanders
I usually try to keep my site more RPG based. But Archaon is just one of those characters... I mean it has been decade (in real life) when the Storm of Chaos campaign was published. Archaon has been the big-bad-Chaos-commander ever since in the WFB books, and the SoC event itself has been "hanging" in the timeline.
Now, finally, there is a book about his story! See more information from the Black Library site.
Also I'm interested in this, because the Norsca Project. And I have heard rumors that more books about his story are coming?!