Let me take a stab at it.

I think the crux of the thing was pride. Horus was the Emperor's favorite son, chosen Warmaster of his armies. Horus led the emperors armies across the stars concuring in his name. After a while Horus started getting kinda pissed off that he was basically doing all the hard work while the Emperor stayed on Terra and claimed the glory.

At some point Horus fell to a poison created specifically to kill Primarchs (Nurgle) which eventually overcame his system. In a desperate attempt to revive their fallen Primarch the 'Sons of Horus', later re-named the Black Legion I believe, took him to a shrine world and locked him in a serpent temple of some kind. One inside the forces of the Old Gods corrupted and revived him. A servant of Chaos (one of the first traitor marines) showed him a portion of the Future where the Emperor is the GOD-Emperor and Horus and certain other Primarchs are not represented (because they turned to to Chaos, ironic huh) and he makes his own conclusions. You can sorta see what happens after that.

 

I'm not sure about all the nurgley poisons and stuff, where is that one from? it really depends who you believe, I think. There's the new black library version, the sabertooth version, the old version, the even older version.. It's been chopped and changed around by various conflicting sources quite a lot.

Generally, though, in all of the different versions, he gets sick of being sent off on horrible bloody campaigns to expand the Empire while the Emperor sits on Terra feeding sweeties to Rogal Dorn. He experiences the power of chaos, somehow, and realizes that the Emperor is betraying mankind by accepting weakness and refusing to embrace the power of Chaos. He resolves to cast the Emperor down and become a new Emperor, leading humanity into a golden age forged by the power of the warp.. and.. yeah.. the rest is history.

As for the other chaos marines, here's a brief rundown of the legions I've read anything about.

Lunar Wolves/Sons of Horus/Black Legion - They were incredibly loyal to Horus and most of them immediately adopted his vision anyway.

World Eaters - They had brain surgery to make them more aggressive in battle, but they also became increasingly bloodthirsty and addicted to the thrill of the slaughter. In the end, they just cared more about killing than about who they killed for.

Death Guard - Typhon/Typhus, who was a chaos worshipper long before the heresy began, sabotaged their flagship and left them stranded in the warp. The marines on board all contracted a warp bourne disease. In desparation, they turned to nurgle to prolong their lives, and were reanimated as plague marines.

Emperor's Children - They were obsessed with being the most perfect warriors in the universe, but this meant they were extremely proud and unhesitatingly loyal. Fulgrim, their primarch, was a close friend of Horus, and when he turned the rest of the legion followed him rather than the Emperor. They were also given a vision of a perfect galaxy in which evil could not exist because nothing was forbidden.

Thousand Sons - They dabbled in forbidden sorcery, believing they could use it to serve the Emperor's plan. When Magnus the Red used forbidden sorcery to warn the Emperor of Horus' impending treachery, the Emperor, who trusted Horus abseloutely, assumed Magnus was the traitor and sent the Space Wolves to destroy the Thousand Sons legion utterly. During the battle, Magnus' psychic wards broke and he was driven insane by the power he sought to control.

Iron Warriors - They fought the toughest, most brutal attrition battles of the crusade, but often weren't rewarded for it. They felt slighted by the Emperor, particularly since he seemed to personally favour the Imperial Fists over them. Horus played on their paranoia and bitterness, and eventually it got the better of them.

Word Bearers - Their entire purpose was the veneration of the Emperor as a living God. When the Emperor rebuked them for it, and rejected their worship, it utterly destroyed everything they'd based their lives upon. In the resulting crisis of faith, they turned to chaos, who actively sought their veneration and rewarded their faith.

Night Lords - None of the other Primarchs liked night haunter, which isn't surprising, he was a derranged self hating freak with a tendancy to emo out and attack them (half joking there.) That, and his legion were a bunch of morally bankrupt sociopaths in power armour.

Alpha Legion - Abseloutely no idea.. Maybe someone with the right book can fill me in?

 

Alpha Legion - Alpharius and Horus had continually been comrads despite their rocky introduction to each other in which the uninitiated Alpharius and his brethren had attacked the Sons of Horus. Before a duel between the two would meet any conclusion they came to recognize one and other for who they were and made amends. However the Emporer didn't give much time to Alpharius and the other primarchs, particularly Guilliman didn't have any respect for him (nice to see Guilliman being the reliable **** he is as normal)

Anyway, it was basically out of this lack of respect for Alpharius as an underrated tactician by everyone but Horus, which ultimately led to his, and thus the Alpha Legions switch of Alliegience.

I think that's right. And I also believe SteveSane is correct two about his analysis. The Priest was actually a chaplain of the Word Bearers, called Erebus who already secretly praised the ruinous powers. Though it wasn't a potion, Horus had been stabbed by a poisonous weapon wielded by the leader of a traitorous cult who praised Nurgle.

 

Read the Horus heresy serious, it explains everything.

 

Thanks for the info u guys its been on my mind for a lond time and if anyone else can contribute to this forum it will help also:yes:

 

The official fall of Horus, as I understand it (given that I've not read Abnett's new series, and utterly detest the Sabretooth fluff) is as follows.

1) Horus falls ill.
2) He is recuperated in a warrior-lodge on Davin, which he was in the process of introducing into the Imperium. Nothing that unusual, Horus was famed for "going native" in order to bring worlds into the Imperial fold.
3) He starts introducing warrior-lodges into the Sons of Horus as part of their combat doctrine.
4) He turned to Chaos.

The speculation is that the warrior-lodge he found was a Chaos cult, and they converted him. How, I'm not sure. It's also speculated that given the Primarchs' brilliant immune system, that the illness he caught had to be Chaos-borne as well.

 

Horus fell ill becuase he was stabed by a plague sword. At Davin, a ritual was preformed on Horus by the chaos cult. The luna wolves, although knowing the cult was chaos, was desperate enought to do anything to save thier primarch. Any way, Horus had this vision induced by the ritual, where the chaos gods pretty much tricked him into believing the emperor had betrayed him and only cared about himself yada yada yada... therefore, the chaos gods gave him the choice to surrender himself to them, and since he believed the emporor had betrayed him, he allowed this to happen to himself.

Its all in the horus heresy series. Its a good read. Read it.:yes:

 

Yeah I can remember reading Xerxes version somewhere. That in order to get a new planet to join the Imperium they asked him to join their cult of warriors or something like that. He did so but the cult turned out to be chaos and slowly corrupted him (in secret of course).

And then he left with a hint of chaos in him and thats all it took...

 

someone told him chaos looks cooler and has hairpieces and he couldn't resist, oh and he was drunk, really really drunk...

 

I like your version best techmarine Rannon.

If you're trying to find basic fluff about it a good place to start is here. I find it quicker for finding out widespread stuff rather than searching through all the books again. And for those who think anything's wrong then source it and change it yourselves.