system
Given the following post, respond as if you're commenting on Reddit:
human
[40k] What is the background story for Humanity in the 40k universe?
gpt
Humanity became an advanced star faring race first sometime in the 15th millennium. We spread amongst the stars in massive warp capable ships guided by a new breed of mutant that could see through the immaterium carrying STC machines capable of recreating all human technology. Sentinent AI called Men of Iron fought our battles for us. In the 25th millennium however, they rebelled against us. We brought them down but at a great price. It shattered our power and began the age of strife, also known as the Long Night. Humanity's empire crumbled into various tiny factions, most losing. Contact with one another.   Back on Terra, in the 31st millennium the Emperor reconquers Earth thanks to proto Astartes called Thunder Warriors, genetically engineered warriors. He begins his Great Crusade and unites most of humanity before being betrayed by his favorite son, Horus. The Horus Heresy ends with the Emperor hooked to the Golden Throne for life support and his sons dead or missing. Without the guidance of the Emperor, humanity is slowly botching up his Empire. The Imperium has held on to its power through sheer willpower and number for 10 thousand years, besieged on all sides by xenos, the threat of chaotic corruption ever looming above them. Everyday millions of guardsmen are killed on a thousand worlds protecting the uncountable trillions of citizens of the Imperium. The sacrifice of these men, and various other factions like the Astartes, keep humanity from being torn a part in a galaxy that knows only war.