[Beastie Boyz]

Actually, in all...

The Word Bearers, in the beginning of the Great Crusade, were advancing slowly, unlike the other Legions. This was because they methodically punished each and every planet for not zealously worshiping the Emperor. He put a stop to all of that, and they advanced at a faster pace.

Later, Kor Phaeron, a Lieutenant of the Word Bearers, and best of friends with the Primarch, Lorgar, of the Chaos Gods. Specifically, how they welcomed, even demanded zealous worship of them, and rather than worship one, they worship/ed the entire Chaos Pantheon. Lorgar appointed Phaeron Master of Faith, given the task to turn the entire Legion to Chaos. As someone else had stated, they had turned to Chaos before the Horus Heresy.

While on a diplomatic mission to the Interex, a small empire consisting of (If I recall right) around 20 planets, with valuable STC information. They were a radical empire, comprised mostly of Humans and another race called the Kinebrach that were a dying race, which had wisely allied with the Human Interex. As such, some, like Abaddon, were wary of the Interex, and saw no point in trying to negotiate.

Now, the Interex were different from the Imperium of Man. The Eldar had told them of Chaos, and of the Warp. They were very wary of Horus and his men from the beginning. Considering his title Warmaster, the Interex was afraid the IoM was tainted by Chaos. Only when Captain Garviel Loken, 10th Company of the Sons of Horus conversed with Mithras Tull of the Interex was it revealed to Tull that the IoM were ignorant of Chaos! Tull explained to Loken how they had feared the Imperium for fear of a Chaos taint. Right after that it was reported that a weapon had been stolen from a museum that the Interex's diplomatic party had shown Horus' later. The IoM's diplomatic party had to fight with the Interex to escape, as they wanted all Marines to be disarmed and put in lockdown, with all vox systems being blocked. Horus was down on the Interex planet, and so every Marine was concerned for his safety.

Later in the Horus Heresy series, in False Gods is it revealed that Erebus stole the weapon, a Anatheme, which is a sentient weapon forged by the Kinebrach. The weapon was then given to Eugan Temba, a great warrior left by Horus to govern a inglorious planet named Davin. He was severely saddened by being left behind by Horus, and it turned to anger, but that isn't shown in the series. Instead, when the speartip landed onto Davin, all that was shown of Temba was that he became a worshiper of Nurgle, as had the various citizens and units on the moon. Whether or not the Word Bearers had turned Temba into the rotted living corpse that he was described as isn't known. It was then that Temba and Horus were mono e mono, because of all the fog, a vox block, and a explosion separating Horus' entourage with him. Temba had given Horus the unhealable cut on his shoulder with the Anatheme before being killed.

A host of Space Marines had found Horus, and he was fine for nearly ten minutes. Just like that, he fell flat on his face, and was rushed to the Vengeful Spirit. Apothecary Vaddon tried to help him, but it was useless. Nothing worked against the Anatheme's cut, even Horus' super-immune system. So Erebus had him brought to the Lodge of the Serpent, a healing lodge on Davin. Someone stated earlier that it was revealed the Lodge was a group of cultists, which is correct. Erebus appeared in Horus' 9 day dream, in which he was to decide his fate. Erebus appeared as Hastur Sejanus, a man who had fallen in a previous battle who was one of Horus' friends. In the same time span, Magnus the Red, the Cyclopean Primarch, foretold that this would happen. He used sorcery, something that was banned by the Emperor in the Council of Nikaea, to gain access into Horus' 9 day dream. The three, Erebus diguised as Sejanus, Horus, and Magnus were discussing things...

Magnus revealed that Sejanus was Erebus in disguise, who in turn told Horus that Magnus used sorcery to gain access to his dream, which greatly angered Horus. Magnus tried to convince Horus to listen to him, that Erebus was trying to turn him to Chaos, but Horus wouldn't listen to his brother primarch, nor would he listen to Erebus for deceiving him. In the end, all the discussion did was further Horus' doubts of the Emperor, and feed his anger towards Magnus (who later turned to Chaos with his Legion in a twist of irony). When Magnus' efforts with Horus had failed, he turned to the Emperor.

Magnus, using a sorcerous ability, contacted the Emperor and forewarned him about Horus' betryal. But the Emperor was more furious for breaking the Edicts of Nikaea, and as such backed Horus against Magnus' proposal. The Emperor told Leman Russ to bring Magnus back to Terra, so that they could talk about Magnus' actions. On the trip to Prospero (Magnus' world), Horus contacted Russ and persuaded him to outright destroy Magnus. On Prospero, Magnus was beaten by Russ, but his sorcerous powers once again saved himself, his Legion, his world, and the knowledge he had accumulated by having a deal with Tzeentch. All of the aforementioned things were moved into the Eye of Terror, and the next time the Thousand Sons were to be seen, they would be on the side of Chaos.

I'll stop my rant for now. If anybody wants me to continue, ask! I'd be glad to at another time.