system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: The Warhammer 40000 Universe

Specifically how the chapters came about. Im kind of a fair weather fan of Warhammer. Enjoy what little of the lore I understand. Play the videogames. Sorely tempted to buy and paint some. But too "proud" (replace with overwhelmed, bashful, shy) to actually enter a Games Workshop. 

Today I was playing Dawn of War II and was delighted to find out you play Blood Ravens in the campaign, my favourite chapter. 

However even though Ive read the wiki article I don't understand how the chapters came about. Something about the emperor sending out people who mutated and something something...

So explain like I was 5 please.
gpt
At the beginning of the 29th millennium (year 28,000), the Emperor, an immortal being, perfect in nearly every way, began a project to create for himself 20 sons. These 20 sons, the Primarchs, would each embody a particular aspect of the Emperor's perfection.

The project nearly came to fruition, but for the intervention of the Chaos Gods, four representations of humanity's darkest emotions. Their powers scattered the 20 children across the galaxy, each to a separate planet, to be raised in different ways.

After the scattering of the primarchs, the Emperor decided to launch a Great Crusade, to reclaim the galaxy for humanity, to reunite the planets separated by 7000 years of darkness and war, and to find his sons in the bargain. To this end, he created 20 Legions of genetically-enhanced super soldiers, the Space Marines. Each of these legions were thousands strong, and each Space Marine's enhancements came from the genetic code of their primarch. 

As the 30th millennium, and the Emperor's Great Crusade, raged on, the primarchs were found, one by one. The first to be found, and soon to be the Emperor's favorite of all, was Horus, primarch of the 16th Legion, the Luna Wolves (soon to be renamed the Sons of Horus). Horus most closely embodied the perfection of the Emperor. The last to be found was Alpharius, primarch of the Alpha Legion, and he embodied the Emperor's secretive nature. There were, however, two primarchs about which very little is known. Their Legions have been struck from Imperial records, and the primarchs themselves were forbidden to speak of their unknown brothers.

In the beginning of the 31st millennium, Horus found himself lured to the Chaos Gods, and convinced eight other primarchs, and their Legions, to rebel against the Emperor in what would be known as the Horus Heresy. At the end of the Heresy, three primarchs lay dead. Horus himself was obliterated by the Emperor's own hand. The cataclysmic duel between Horus and the Emperor would force the Emperor to be enshrined in the Golden Throne, a life support machine powered by the souls of humans.

Shortly after the Emperor's Ascension to the Throne, Roboute Guilliman, the primarch that embodied the Emperor's tactical genius, wrote the Codex Astartes, a book that defined tactics and organization for the Space Marines. Shortly after the completion of the Codex Astartes, the nine loyal Legions were broken down into forty one-thousand-strong Chapters of Space Marines in the Second Founding. Nine of these chapters retained the names and insignia of their founding Legions.

Since that time, there have been at least 24 other Foundings of Space Marine Chapters, as chapters wax, wane, and are destroyed altogether. The great majority of Space Marine Chapters can trace their genetic lineage back to a founding Legion, one of the original nine, and this lineage defines many aspects of a particular chapter, including the level at which they follow the Codex Astartes, the value they place upon different aspects of war, and even some genetic traits, such as the Salamanders' coal-black skin or the Blood Angels' Red Thirst.

The Blood Ravens in particular are notable because they have no knowledge of their primarch, and as such, have no chapters that they can truly call "brother." This is extremely rare, for a chapter not to know their primarch, and is a source of great frustration for them. The Blood Ravens therefore value knowledge above all else, as there must be someone in the galaxy that knows the identity of their primarch.

I think that should do it...