Legions

The formation of Legions date back to the 30th millennium, the Age of Strife, where the fall of galactic nation is coulpled with Terra becoming an uncivilized battle ground of bickering minor warlords. The Emperor of Man appeared as a messianic leader who seeded to retake the galactic empire. To do this, he created 20 more super-human generals to lead the existing army of super-human soldiers. These generals would poses some of Emperor's genetic code and good amount of his psychic powers. This was called the Primarch Project. However, the infant Primachs disappeared from the secrect base on Luna. Feeling the the Primarchs were still alive, the Emperor used the progress he had made with the Primach Project and made 20 units of more advanced, genetically engineered super-soldiers. Each unit was created from the DNA of one Primarch. These units would number from 10,000 to 15,000 Marines and would be called Space Marine Legions.

As the Emperor and the early Imperial Army regained the the galactic empire through the the Great Crusade, the lost Primarchs are found and take command of their respective Legion. Along with a Primarch, the Legions gain names.

It is after the Horus Heresy that the Legions are broken up into smaller Chapters to help prevent future treachery.