He never wanted to fall.
Archaon's true story is that he was born of the rape of a fisherman's wife by a Varg marauder, she died in childbirth and the family refused to keep him but they wouldn't kill him either. A midwife left him on the doors of a monastery where he was raised by Father Hieronymus Dagobert who gave him the name Diederick. When Diederick turned 13 he wanted to be a knight so Dagobert got him a page position with a knight called Kastner, who was a drunken womanizer who sired bastards like some men breathe. Kastner eventually died in the Drakwald and Diederick went to the man's wife who was a genuinely good person and made up a story about being his bastard, the wife often sponsored his bastards and let them carry the Kastner name. Never said whether she really believed him or not but she let him become Diederick Kastner and helped him enter the Order of the Twin-Tailed Orb.
After some time Kastner fights against a small warherd and takes a shard of warpstone to the eye, which begins to change him. He also saves Giselle Dantziger who is fleeing the destruction of Hammerfall, a depository of forbidden lore, and is carrying Liber Caelestior, or The Celestine Book of Divination by Battista Gaspar Necrodomo. On the way to return the book they are attacked by a group of Knights of the Fiery Heart who claim the Grand Theoginist has sent them to end Kastner who is the prophesised Lord of the End Times. Before they can do so a group of winged Chaos Knights arrive and help Kastner, Giselle and Dagobert escape to the Gruber Marches, Kastner's home but find that everyone there has been killed by the Fiery Heart, including Angelika Kastner who was the original Kastner's wife. Kastner doesn't want to believe that he is going to be evil, so he goes to Altdorf and sneaks into the Grand Temple of Sigmar to confront the Theoginist. The priest reveals that the true Archaon is the one who will come in search of his name, which Kastner has done. Kastner prays for Sigmar to help him, but gets no answer and after realising that no answer is coming, curses Sigmar and says that he will destroy the Empire and Sigmar's people in revenge for Sigmar's refusal to help his loyal son.
Kastner escapes Altdorf with the help of the winged knights who return for him, and with them and Dagobert and Giselle in tow he heads for the Wastes, and accepts the name Archaon.
So that is how Archaon was made. Very surprising stuff and a departure from what we thought happened. He did read the prophecies of Necrodomo but they didn't drive him insane and he wasn't the one who murdered his family, although he did torch the home when he left for the Wastes. I imagine the Twin-Tailed Orb would see to it that his membership was stricken from all historical record, and the Empire made up a version that didn't make them look bad. Not surprising, when you think about it.
Archaon's wish to not fall was very surprising though. He wanted to be a loyal Sigmarite, he even begged Sigmar to help him fight against the destiny being imposed on him, and all the denials he made about being evil and wanting to be good really was not what I expected from the future Lord of the End Times. It's weird to think of Archaon as a victim but he is, he never chose to fall, he had it forced on him by the Gods. Eventually he did become evil but that was more going along with the flow rather than an actual decision made with free will, he was going to become Archaon no matter what and when Sigmar does not answer his prayers he decides to destroy Sigmar and all the Gods, and if he has to be the Everchosen to do that, then so be it. Archaon isn't just a destructive monster, he's an Omnicidal Maniac who wants to destroy everything until only oblivion is left, that is what the Lord of the End Times is truly meant to do in his mind. Scary.
What's truly surprising though is the amount of times Archaon dies in the book, and every time Be'lakor either reverses time or tweaks something to change events, not sure yet (Asked Rob, hopefully he can shed light on that) but it shows how much Be'lakor has messed with events to make sure Archaon survives and follows the correct path. His deaths include;
-Never being conceived as before the Varg can rape his mother her husband kills him. Be'lakor changes things and the shrine bells don't ring in the attack, so the fishermen are not alerted and the husband doesn't return in time.
-Being killed at birth when his mother gives birth on the beach and gives the child to the sea. Not sure how Be'lakor tweaks things this time but he does.
-Dying of a plague at age nine, Be'lakor twists it so he never got sick.
-Getting kicked in the head by a horse at age 13, not sure how Be'lakor fixes that but he does.
-Dying of the wounds he sustained killing Beastmen, I think Be'lakor made it so that the attempted treatment didn't kill him.
-Hanging himself rather than become Archaon, I think Be'lakor gives him a little more will to keep going to prevent that.
-Being poisoned by Dravin Vayne and his sorceress lover Sularii, not sure how Be'lakor tweaked that or what exactly happened in that few chapters (I've asked Rob about it.)
-And being eaten by Flamefang without anything to protect him, Be'lakor actually forces him to keep going.
Be'lakor seems to be able to affect the flow of time and he keeps changing things to make sure that Archaon lives and becomes the Everchosen, so that Be'lakor can have his body obviously. Makes for a very interesting narrative as Archaon dies and you wonder "what the hell?" only for the next page to be Be'lakor narrating and explaining the cause and effect theory, destiny and offering commentary on what Archaon has done or has had done to him. And the next chapter, titled the same as before, continues as if the death and what caused it did not happen. Really interesting aspect which made things unpredictable.
One thing this novel had that I really enjoy in all BL works is a lot of names, the names of Chaos warriors and their armies that evoke imaginative images and lots of insane ideas. Examples;
-Mother Fecund and her Maggot-Men
-Fengshen Ku and the Dreaded Wo
-The Bronze Company
-The Brothers Spasskov and the Fleshstorm
-Dravin Vayne and his Slaaneshi Corsairs
-Escoffier the Mad Bretonnian
-The Great Spleen and the Ravening
-Ogvaldr the Aesling and the Daemon Blade Snaga
-Nikitia and the Annointed
-Bhlorgl the Obscene
-Grastlana le Faux
And many more. I like the variety and images that these names evoke, each one is distinct from the others and creates the idea of a vast horde of entire different armies that are marshaled together by the will of a singular individual like Archaon.
And the Swords of Chaos who never speak and so Archaon names them Eins, Zwei, Drei, Vier and Fünf. Despite never speaking or having personalities, these five do make for interesting characters by their actions and the small little things that give glimpses into who they might have been once. Their naming was very amusing, especially that Archaon drew numbers on their heads to tell them apart. And when Vier fist-bumped Archaon's shoulder before going to fight Flamefang despite the mutations that have near-crippled him, that moment made me laugh in a nice way. Vier was a hell of a bodyguard then, and that was the most personality that any of them showed in the entire book. Very glad he survived.
Best antagonist in the book: Flamefang. The Yien-la-long. The Terror of Grand Cathay. A true Daemon-Dragon. Flamefang's spawn-flame was very devastating but what made him truly memorable was his appearance, a dragon made of stolen flesh. Everyone he eats gets added to his body and makes him bigger, his skin covered in protruding eyes and faces that wail, his insides the same. And I LOVED the reference to Alien when Flamefang lets out a second head from inside his mouth.. and then a third one. Very disgusting image, made for great reading.

I also really liked the Southern Wastes, never knew they existed before this. The darker side of the world where no-one goes, where only Daemons and degenerate monsters live to prey on each other forever, where Chaos truly takes form in that no-one exists to bring sanity there, and that another doorway exists there and that if one goes through it, one will arrive at the Northern Wastes. So ultimately as bad as the Northern Wastes are, the Southern Wastes make them look like paradise.
LotN