Primarchs

The Primarchs were the twenty genetically-engineered "sons" of the Emperor of Mankind, and the genetic "fathers" of the Space Marine Legions. The Emperor used his own DNA in their creations, and they were designed to be far superior than any average human. Immensely larger, stronger, hardier, faster, intelligent, and charismatic, than all others save the Emperor Himself, their main role was to be the generals and leaders of the Imperium of Mankind's war machine. Each was made for a specific purpose and specialty, allowing them to serve in specific functions that the Emperor's forces would need in reconquering the galaxy.

Creation of the Primarchs
They were created in a secret underground laboratory on Terra under the tightest security. According to Ingethel the Ascended, the Primarchs were created by the Emperor with the aid of forbidden sciences and arcane lore derived from the Warp. Eldrad Ulthran also states that the Emperor utilized dark powers to construct the Primarchs. It is speculated the Primarchs were imbued with energies and knowledge that the Emperor gained in a Warp Portal on Molech.

However, the Gods of Chaos somehow managed to spirit them away just prior to their maturation with a great Warp vortex and it is speculated that they also managed to tamper with the infant Primarchs.

Scattered across the galaxy, beyond the Emperor's reach, and all landed on different worlds vastly separated from their Father. The Emperor could sense that the infant Primarchs were alive, but was unable to reach them. Therefore 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. The new homeworlds of the infant Primarchs and subsequent experiences on those worlds would influence and mold them to a large extent. There they quickly grew to adulthood and often rose to power, ruling, leading, and helping the inhabitants. As the Emperor found them, he was generally pleased by their actions, and quickly gave them the leadership of their respective legion.

Rediscovery
The first Primarch to be found was Alexis, who became the Emperor's favourite. Most Primarchs somehow recognised the Emperor on sight, and immediately pledged their allegiance on the spot. A few, only followed him after being bested in contests. The last to be found were Nocterra and Solaris.

According to Malcador the Sigilite, both the Emperor and himself deliberately agitated the Primarchs and sought to turn them against one another, with the resulting war purging certain unmanageable Primarchs and their respective Space Marine Legions just as they had the earlier Thunder Warriors. This would leave humanity, not bio-engineered superhumans, as the rulers of the galaxy. However, the Ruinious Powers intervened before their plan could be fully formulated, in turn leading to the Great Secession and the exile of the Emperor's Loyalists. This account may be either a partial or total lie, as Malcador himself seems to keep its validity ambiguous.