There he finds Horus standing over the dead body of Sanguinious, the Primarch of the Blood Angels. Eventually, the Emperor sees his chance to stop Horus and teleports aboard Horus’s ship.After much war across the galaxy, the traitor fleets enter the Solar System, led by Horus, and begin the Siege of Terra.They fought with the Imperial Fists, who had been fortifying the solar system, and the Alpha Legion’s Primarch, Alpharius, is thought to have died. The traitor legions headed toward Terra, with the first–the Alpha Legion–arriving for the Battle of Pluto.The next major betrayal would happen at Calth, where the Ultramarines were attacked by the Word Bearers, and it was thought that the Ultramarines’ Primarch, Robute Guilliman, was killed.This is known as the Drop Site Massacre, and it nearly wiped out the three loyalist legions. One the planet of Isstvan V, three legions–the Iron Hands, the Salamanders, and the Raven Guard–are again betrayed, this time by the other five legions–the Emperor’s Children, the Iron Warriors, the Night Lords, the Word Bearers, and Alpha Legion. Learning of this betrayal, eight Space Marine Legions are dispatched to Isstvan.At Isstvan III, Horus destroys all of the Space Marines who will not follow him into betrayal from the Emperor’s Children, World Eaters, and Death Guard.

Instead, he opens a rift into The Warp inside the Webway, letting daemons loose and ruining the Emperor’s project. One of the Primarchs–one who is the most powerful psyker, Magnus the Red–learned about Horus’s treachery and tries to warn the Emperor via psychic communication.The wound is terrible, and to save himself Horus made a pact with the powers of Chaos. Horus is wounded in battle at the Serpent Lodge on Davin.The Emperor left the Crusade to return to Terra and work on a secret project.

As the Crusade was going well, the Emperor made one of the Primarchs, Horus Lupercal, the Warmaster, who would lead all the others in the Great Crusade.

The Primarchs led their Space Marine Legions in a Crusade to regain the parts of the Milky Way galaxy that had been lost during a previous era, the Age of Strife.

And from each of these he made a legion of the Space Marines, each one having the genetic traits of its Primarch.

The Emperor made 20 Primarchs, kind of demi-god-like beings who are a kind of super, super human.

Horus heresy books in order