He promised to become a knight, but what the heck is Astartes?

My name is Rorschach, a man of absolute loyalty to the Emperor.
An accident sent me to time travel!
Good news! I've been transported to a world of swords and magic, wizards and knights, dwarves and elves. And this world also has a supreme god, and a demon god seated high on his throne!
But the bad news is! This isn't the sacred Terra that Rorschach so desperately wanted to protect with his life!
"Chovy! You should have dressed properly for me when you transmigrated!"
"I want to be a Space Marine, I want to be an Astartes! Who wants to be a knight?!"
Welcome to Warhammer 40
[I must say, you are truly a lucky one. Your absolute loyalty caught the Emperor's attention, and after his death, he summoned you to the world of Warhammer 40.]
This is a world filled with the roar of chainswords, the tremors of psionic energy, interstellar wars, and the rampage of warp gods!
[That sounds dangerous, doesn't it!]
So if you want to save your life, then strive to become an Astartes!
"Huh? Is your system legitimate?"
[My God! The wicked Slaanesh has escaped from the Warp! And you, a warrior of Astartes bearing the weight of loyalty and honor, defeat her! Let the Emperor witness your loyalty!]
[Mission Rewards: Ultramariner Gene Potion x1, Emperor Honor Points x800, Increased resistance to Slaanesh demons.]
Luo Xia swallowed hard as he looked at the exotic black-stockinged witch crawling beneath him.
"Who said this system is lame! This system is absolutely amazing!"
P.S.: [Displacement System] [Invincible] [Ancient Method of Hand-Drawing Immortals]