First Founding
After he conquered Terra, the Emperor of Mankind set out on a Great Crusade to re-unite the lost human colonies into a mighty Imperium. He soon realized that bog-standard humans wouldn't be enough; some situations called for a small number of devastatingly powerful warriors, a surgical strike force to smash enemy linchpins. To fill this role, he created his Primarchs, and from their genes, he created the Legiones Astartes, the twenty Space Marine Legions:
| Legion Number | Primarch | Homeworld | Name of the Legion | Allegiance | |
| I | Lion El'Jonson | Caliban | Dark Angels | Loyal | |
| II | +++Records expunged+++ | ||||
|---|---|---|---|---|---|
| III | Fulgrim | Chemos | Emperor's Children | Traitor | |
| IV | Perturabo | Olympia | Iron Warriors | Traitor | |
| V | Jaghatai Khan | Mundus Planus | White Scars | Loyal | |
| VI | Leman Russ | Fenris | Space Wolves | Loyal | |
| VII | Rogal Dorn | Terra | Imperial Fists | Loyal | |
| VIII | Konrad Curze | Nostramo | Night Lords | Traitor | |
| IX | Sanguinius | Baal | Blood Angels | Loyal | |
| X | Ferrus Manus | Medusa | Iron Hands | Loyal | |
| XI | +++Records expunged+++ | ||||
| XII | Angron | Unknown | World Eaters | Traitor | |
| XIII | Roboute Guilliman | Macragge | Ultramarines | Loyal | |
| XIV | Mortarion | Barbarus | Death Guard | Traitor | |
| XV | Magnus the Red | Prospero | Thousand Sons | Traitor | |
| XVI | Horus | Cthonia | Luna Wolves/Sons of Horus | Traitor | |
| XVII | Lorgar | Colchis | Word Bearers | Traitor | |
| XVIII | Vulkan | Nocturne | Salamanders | Loyal | |
| XIX | Corax | Deliverance | Raven Guard | Loyal | |
| XX | Alpharius Omegon | Unknown | Alpha Legion | It's complicated | |
Horus Heresy
For all their power, the Space Marines and Primarchs were not perfect; half of them were sufficiently flawed to be tempted by (or driven to) Chaos, led by Warmaster Horus. The traitors were defeated, but at a terrible cost, including the near-death of the God-Emperor of Mankind and trillions of deaths.
Roboute Guilliman decided that the so-called Horus Heresy was proof that one man could not be trusted with power over one-twentieth of the Imperial Armed Forces, so he enacted several reforms to divide the Imperial Army into the Imperial Guard and Navy, and split the remaining loyalist Legions into Chapters, in accordance with his Codex Astartes. This event was later known as the Second Founding.
Two Unknown Legions
In the First Edition of Warhammer 40,000, the twenty First Founding Chapters were all known. It seems that Games Workshop had more love for some than others; when they made fluff revisions in the transition to Second Edition, the Valedictors and Rainbow Warriors were demoted to one of the "Later Foundings."
Nowadays, nothing concrete is known about the Legions II and XI, and likewise their unknown Primarchs. Officially, the Imperium deleted all records regarding the "Lost Legions"; the only reminder of the two legions were empty plinths in the Hegemon at the Imperial Palace. Throughout the Horus Heresy series, it is suggested that the Space Wolves destroyed them for some reason. In the book The First Heretic, when a daemon takes the Word Bearers on a trip back in time to show them the creation of the Primarchs, the Word Bearers dialogue indicates that the XI Legion in particular did something bad enough for the Emperor to lead their purge himself, and that the left-over Marines were folded into the Ultramarines (their Chaplain dismisses it as a rumor, though it is public record that the Ultramarines received a major boost in troops at the time). In the book Fear To Tread, Sanguinius tells Horus that he hasn't revealed the existence of the Red Thirst to the Emperor because he fears that the Blood Angels would receive an empty plinth in the Hegemon as well, indicating that gene-seed flaws may have also been a factor. Deliverance lost has a dialogue between Corax and the Emperor where he asks why only sixteen of his brothers were waiting to meet him if he was the nineteenth Primarch to be found, only for the Emperor to deflect the question; consequently, we can assume that the Legions were purged sometime before Corax's discovery (and were never around for the Horus Heresy). He also forced the remaining Primarchs to swear an oath never to speak of their absent brothers, so whatever they did must have been extraordinarily bad.