The Emperor created twenty legions of Space Marines. Two of which are unknown. Officially there were eighteen Primachs, unofficially there was one more, but more on that in the Primarch post. While the Primarch was growing in their pods, the Chaos gods managed to scatter them all over the galaxy. The Chaos gods all require their own posts and an entire series, but for now, I will say that they are extra-dimensional entities that are fighting the Emperor.
The Emperor soon launched the Great Crusade and sent his armies to find the lost Primarchs. The first to be found is Horus Lupercal the Primarch of the Luna Wolves, renamed the Sons of Horus. He soon became the Emperor's favorite son and he was the only one to spend his formative years with the Emperor.
When all of the Primarchs were found the Emperor left Horus as t he Warmaster and the leader of the Great Crusade. He then returned to Terra to continue with his secret project, the creation of the Imperial webway. The webway is something I will explain more when I talk about Eldar, but for now, I can say that it is a separate dimension used for FTL travel. The Imperium uses the Warp for FTL and it has proven as unreliable and dangerous. This is why the Emperor wanted humanity to start using the webway in order to save people and give the much faster and safer form of FTL travel.
What he did not know is that some of his sons have turned traitor. The first among them was Lorgar, who decided to sway others to his side. His main goal was to get Horus to turn against the Emperor. Clever planning and telling the truth in a way to obfuscate some parts of it managed to turn even the Emperor's favorite son against him. Soon Horus Heresy started. Half of the Primarchs turned traitor and some of the loyalists were killed. Horus Heresy is a cataclysmic civil war that ruined the Imperium and the Emperor's dream for humanity, but this is also a story for another time.
While the Emperor was still busy on Terra, one of the Chaos gods managed to trick Magnus the Red, the Primarch of the Thousand Sons to send a particularly powerful psyker message to warn the Emperor of Horus's betrail, but this message had to go through the barriers Emperor set up and managed to pierce the Imperial webway. The tear let a large number of Daemons in. Daemons are a malevolent warp entities who are an extension of the Chaos gods they serve. To stop them the Emperor had to use all his power to block the hole in reality and Adeptus Custodes and Sisters of Silence had to battle all of the entities that entered the real space. This kept Emperor and his closest busy for the rest of the Horus Heresy.
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