Legiones Astartes

The Legiones Astartes or Space Marine Legions, are the transhuman warriors created by the Emperor of Mankind on Terra, following the end of the Age of Strife, to unify the techno-barbarian nations that dominated the homeworld of Mankind and forge the foundation of the Imperium of Man during the Unification Wars. The Emperor began by creating his pseudo-Astartes - the Thunder Warriors. They were only the first step in the creation of superhuman fighters, which finally peaked in the creation of that which would later become known as the Space Marine Legions. The Unification of Terra successfully reunited the ravaged birthworld of Mankind, which rapidly became the Throneworld of the burgeoning Imperium of Man.

The Unification of Terra was only the beginning, and thus, the time of the Terran Wars of Unification was ended and the Age of the Great Crusade was begun. With the union between Terra and the technologically advanced Mars forged, the Great Crusade advanced onward, ready to reclaim the rest of the solar system in the name of the newly born Imperium.

Warbands and Demi-Legions
Additionally, while not legions proper, the following major Astartes forces took part in the Five Ruins: