(Thanks to Peter Griffin, Son of Round Clam, for the bonus update, 12/50.)
Black Rage was a genetic disease of the Blood Angels Astartes line, a Psychic Power-based genetic disease triggered later in life.
Sanguinius, genetic father of the Blood Angels, had been hammered to death by Warmaster Horus!
There was some kind of Psychic Power connection between a Gene-Primarch and his Astartes genetic descendants.
The Blood Angels had watched their genetic father get hammered to death. The pain their genetic father fed back to them before his death instantly flooded their minds, driving the Blood Angels into a rage!
That rage became a Psychic Power genetic disease, passed down through generation after generation.
Specifically, Astartes of the Blood Angels line showed no symptoms normally, but once it struck, the image of Sanguinius being hammered to death by Horus immediately appeared in their minds, and they kept shouting Horus's name.
They instantly lost all sense of pain, while their physical capabilities surged past Astartes limits, driving them to hack their enemies apart in a frenzy!
After killing their enemies, they might even see their allies as enemies and start hacking them down too!
In short, Black Rage drove Blood Angels Astartes mad, making them ignore pain, vastly boosting their physical strength, and sending them on an indiscriminate slaughter regardless of friend or foe!
Once someone fell into the Black Rage, they were basically insane and beyond saving.
Saint Raiser had been chatting with Li Qinwu when he heard someone call Horus. The most sensitive nerve in his brain was instantly strummed like an electric guitar.
"Fuck!! Black Rage!! Everyone, get out of the way!!"
Saint Raiser's mechanical body, which looked very much like a StarCraft Marine, rushed forward. With a tiger-like pounce, he flew over and pinned Recruit Aman down!