When the Emperor set out to re-unite the lost tribes of Mankind at the very dawn of the Age of Imperium, he knew that he could only do so with a mighty corps of warriors at his side who were as strong, skilled, and dedicated as he. To this end, he created twenty perfect beings in his own image—the Primarchs—and from the genetic legacy of each of these he created an entire legion of superhuman warriors.
The Space Marines of the Adeptus Astartes are the inheritors of the original Primarchs, whose blood still flows in their veins. Their bodies enhanced by implants that boost their abilities far beyond the human and their hearts steeled by unmatched courage and devotion, it is truly said of the Space Marines that they know no fear. They are the Angels of Death, bearing the most formidable of weapons and wearing the strongest of armours. Such blessings come at a cost however, for the Space Marines are recruited at a young age and know nothing other than war before eventually and inevitably falling in battle against the endless foes of the Imperium. They are at once far more than human, yet removed from Mankind, their entire existence one of sacrifice, duty, and death.
Chapters of the Adeptus Astartes operate apart from the bulk of the Imperium’s power structure. Each is led by a Chapter Master who is considered a peer of the Imperium, an equal to the highest-ranked servants of the Adeptus Terra. They may be petitioned for aid against invasion or insurrection, or to spearhead the many wars and crusades launched against the foes of Mankind, but they are a power unto themselves and may very well reject such a plea with no reason given as to why. Though it is said there are only a scant million or so Space Marines, the speed with which they attack—and the devastation they cause—makes them rightly feared across the galaxy.