For a list of Primarchs, see Primarch.
The Primarch Project was part of the Emperor's plan to create an army of super-human warriors. The Emperor had already been fighting on Terra with genetically modified warriors, but this project would be his greatest attempt to create generals to lead these warriors, second only to himself in skill, ability, and sheer power. It was begun in M29 with the aim to create 20 cloned 'children' of the Emperor. The facility to create these Primarchs was constructed on Luna and the Emperor oversaw the project, having them grown in accelerated gene culturing chambers. His scientists watched over the growing foetuses in their chambers, afraid of what would happen if anything went wrong. Unfortunately, all the planning and effort that went into creating the Primarchs was almost lost as they were whisked away by a Chaos portal created directly above the facility. This may mean that the Chaos Gods discovered something about these mighty warriors, and been afraid of them. However they were unable to destroy them managing only to scatter them across the galaxy, beyond the Emperor's reach. The Emperor could sense that they were still alive and would search for them during the Great Crusade.