The Legiones Astartes
“They shall be my finest warriors, these men who give of themselves to me. Like clay I shall mould them, and in the furnace of war forge them. They will be of iron will and steely muscle. In great armour shall I clad them and with the mightiest guns will they be armed. They will be untouched by plague or disease, no sickness will blight them. They will have tactics, strategies and machines so that no foe can best them in battle. They are my bulwark against the Terror. They are the Defenders of Humanity. They are my Space Marines and they shall know no fear.” – The Emperor of Mankind
The Adeptus Astartes (commonly known as Space Marines) are the most elite and feared fighting forces of the Imperium. The primary unit of organisation is the Legion, a self-contained army fully equipped with its own transport, non-combatant support staff, etc. There are eighteen Legions, each comprising thousands of Space Marines.
After the infant Primarchs were scattered across the galaxy, beyond the Emperor’s reach, he used their previously collected DNA samples to create the Space Marines. The genetic material of each one of the Primarchs was used to found a single Space Marine Legion, twenty in total (though only eighteen remain).
As the Emperor gradually found his lost Primarchs, he quickly gave them the leadership of their respective legion.
The first Primarch to be found was Horus and the last to be found was Alpharius.