I get very confused about all this Primarch & Emperor fluff. Who are the Primarchs? And what is the Emperor, and what's his backstory? I could look on the wiki, but you guys can probably give me a nice condensed version. WH40K is awesome just for the fluff.
Okay... Lemme take a crack a crack at it.
Around 30000 A.D., this guy known to history only as The Emperor (DA EMPRAH to his friends), who may or may not be Jesus, or Longinus, or every important figure in history, conquered the neofeudal tribes of Earth, then the Mechanicus Priests on Mars, and took such a shine to conquering that he went after the whole galaxy. Needing an army to carry out his serious business, he created the Space Marines, genetically engineered super soldiers who could kick ass and take names for him.
To lead these armies, Da Emprah sort of cloned himself (the specifics aren't really clear), creating what would be the 20 Primarchs. Except
SOMETHING happened to the Primarchs that scattered them across the galaxy as infants. Over the next whole bunch of years, 18 of the 20 were found, all of them living legends on whatever planet they grew up on. Each of the Primarchs was granted command of his own Space Marine Legion, impressing upon it their personalities, and their genes (which were only sort of related to Da Emprah's) were the origin of the Space Marine genelines.
Foremost of the Primarchs was Horus (real original I know), lord of the Luna Wolves (not to be confused with the Space Wolves). Horus was Da Emprah's favorites son, and led all of his personal battles. Naturally, Horus turned out to be a titanic dick who wanted the Imperium for himself, and turned to worshiping the Chaos Gods at some point (when, how, and why differs with every account). When he went all Oedipus Rex on Da Emprah, he convinced half of the Primarchs and their legions to go along with him, massacring most of the Loyalists in one swoop and laying siege to Earth.
Eventually, in a battle that's been rewritten too many times and ways to count, Horus slew Sanguinius, Primarch of the Blood Angels, before being slain in turn by Da Emprah himself. However, the battle left him also mortally wounded. Thinking quickly, his closest servants chose to preserve him within The Golden Throne, a holycrap arcane... thing... that would both keep Da Emprah alive forever (
but not really,
well sort of,
except... it's complicated), and also turn his already superhuman psionic power into a mental lighthouse that lets ships navigate the Warp, among many other perks.
However, and it's a
big however, he's pretty much dead for all intents and purposes (
if not really dead HERESY), so he doesn't actually rule anything. Instead, the Imperium is ruled by a bunch of powermongering Illuminati and Generals and Space Popes and such, collectively known as the Administratum and Ecclesiarchy. Basically, all the worst parts of the Chinese government, the Nazi war machine, the Catholic church, and the Aztec empire, cranked up to 11. About 3000 years after he was
not really killed, the Imperium fought a big civil war over exactly how to worship Da Emprah's corpse. Then they did it again 3000 years later. And sort of again 3000 more years later. Which brings us to about the present day of the 41st Millennium.
Meanwhile, those Marine Legions that turned to Chaos, and a bunch of other nasties, all lead by Horus' lieutenant Abaddon the Sore Loser, have launched 13 Black Crusader over ten thousand years, scaring everyone shitless but always accomplishing exactly nothing. And the Eldar kneecap everyone to save their ancient hides, those plucky Tau are about to be eaten by Space Bugs, magic Terminator skeletons want to eat everyone's souls, and the Orks have been a pain in everyone's asses since time immemorial. But I'm sure you knew that.
And now you're hopefully up to speed. If you need any more clarification, feel free to ask.