Primarchs
Origins
The term 'Primarchs' is derived from the Latin words 'primus' and 'archon', which translate to 'first ruler' or 'chief'. In the context of the Warhammer 40,000 universe, the Primarchs are the twenty genetically-engineered "sons" of the Emperor of Mankind, and the genetic fathers of the Space Marine Legions. The Primarchs were scattered across the galaxy, and each was eventually found and reunited with their father, the Emperor.
Creation
The Primarchs were created by the Emperor in the latter stages of the Unification Wars on Terra. Using his own genetic template, the Emperor imbued each Primarch with specific aspects of his own nature, including his psychic might. The Emperor intended the Primarchs to be the commanders of his Great Crusade to reunite the scattered and isolated human colonies of the galaxy under a single, unified Imperium.
Scattering
Before the Primarchs could fully mature, a cataclysmic event known as the Scattering occurred. The forces of Chaos, perceiving the threat the Primarchs posed to their dominion, managed to breach the Emperor's laboratory beneath the Himalazian (Himalayan) Mountains. The infant Primarchs were cast through the Warp and scattered across the galaxy. The Emperor was unable to prevent this disaster, and the Primarchs were lost.
Rediscovery
Over the course of the Great Crusade, the Emperor and his Space Marine Legions discovered each of the lost Primarchs on various worlds across the galaxy. Each Primarch had grown to maturity on their adoptive world and typically risen to a position of power. Upon being rediscovered, each Primarch was given command of the Space Marine Legion created from their genetic material.
The Horus Heresy
The Primarchs played a central role in the cataclysmic civil war known as the Horus Heresy. Horus Lupercal, Primarch of the Luna Wolves (later renamed the Sons of Horus), was corrupted by Chaos and rebelled against the Emperor. Nine of the Primarchs and their Legions sided with Horus, while the remaining loyalist Primarchs fought to defend the Imperium.
Post-Heresy
In the aftermath of the Horus Heresy, the surviving loyalist Primarchs either disappeared, were mortally wounded or ascended to become Daemon Princes of Chaos. The traitor Primarchs, now fully devoted to the Chaos Gods, retreated to the Eye of Terror, a region of space where the Warp and reality overlap.
Legacy
The Primarchs left a lasting legacy on the Imperium and the galaxy. The Space Marine Legions, later reorganized into Chapters, continue to defend the Imperium using the gene-seed of their Primarch. The traitor Legions, now known as Chaos Space Marines, continue to wage war against the Imperium under the banner of their Daemon Primarch masters.