The Space Marines or Adeptus Astartes are foremost amongst the defenders of humanity, the greatest of the Emperor of Mankind's warriors. They are barely human at all, but superhuman; having been made superior in all respects to a normal man by a harsh regime of genetic modification, psycho-conditioning and rigorous training. Space Marines are untouched by plague or any natural disease and can suffer wounds that would kill a lesser being several times over, and live to fight again. Clad in ancient Power Armour and wielding the most potent weapons known to Man, the Space Marines are terrifying foes and their devotion to the Emperor and the Imperium of Man is unyielding. They are the God-Emperor's Angels of Death, and they know no fear
The Astartes are physically stronger, far more resilient and often mentally far removed from the lot of most normal human beings. In the presence of the Astartes, most people feel a combination of awe and fear, and many cultures on the more primitive worlds simply worship them outright as demigods or angels of the God-Emperor made flesh. They should feel so, for many Space Marines feel little compassion for those they have sometimes termed "mortals" in comparison to themselves, seeing the very people they were created to protect as little more than obstacles to a more efficient eradication of the Imperium's enemies. This is an attitude sometimes taken by whole Chapters. They see normal humans as frail, weak creatures given to the follies of temptation, avarice, greed, lust and cowardice -- all emotions they rarely feel, if ever. Yet there are some Astartes who remember why they were created by the Emperor, who avoid the trap of hubris which the Space Marines are so prone to and which has seduced so many of their number to serve the Ruinous Powers of Chaos.
They are the final guardians of Mankind, the saviours of last resort. They were intended not to lead humanity, but to defend it, sometimes even from itself. At the heart of that mission lies the limitless compassion the Emperor extended to every man and woman in the galaxy when he willingly chose to condemn himself to more than 10,000 years of imprisonment within a dying prison of flesh for their sake. Some Astartes sneer at compassion, seeing it as one more human weakness that has been purged from their superior bodies and minds. But the wisest of the Space Marines know that in the end compassion is their only salvation, as it is for all men. In that, they have proven themselves to be fully human.