Horus Heresy
One of the single biggest events in Warhammer 40,000 fluff, second only to the giant orgy of the Eldar that created a Chaos God of pleasure and decadence. This is when nine Space Marine primarchs rebelled against the Emprah, along with numerous Imperial Army forces, under the banner of Horus who was worshiping Chaos. The battle eventually waged onto Holy Terra in a great siege of Grimdark proportions where it was virtually impossible for both parties to win this way and so was settled when Horus and the Emprah had a duel to the death.
The Emperor won against Horus but was fucked up to the point where he had to be permanently attached to a life-support machine known as the "Golden Throne" just to survive. In his absence, logic within the Imperium gradually decreased and thus, eventually turned into the Grimdark empire it is today.
Warhammer 40,000 Fluff
The Horus Heresy screwed almost everyone's plans and changed the flavour of the Imperium's Grimdark from Stalinist Soviet "if you breathe a word about religion, we rape you with knives" to Catholic Inquisition "if you breathe a word about the wrong religion we rape you with knives".
The Board Game
First published in 1993 by Game Designer's Workshop, it was the Emprah versus his evil bastard of a son in the scorched earth of Terra. Units include titans and Chaos spawn. *GLARBLBLBLBLBLBL*
The more recent edition (2010) is published by Fantasy Flight Games. Also a two-player war game, it includes over 100 sculpted minifigs, sculpted buildings, and even Horus and the Emprah themselves are units on the board. It also adds more territory, as the fight can be pushed back onto the traitor's flagship Vengeful Spirit. Combat is less dice-y and more card-y.
(Not to be confused with the lame Horus Heresy card game)
The Book Series
For the last few years, Black Library has been publishing novels that explore the events of the Horus Heresy, looking at the rivalries among the Primarchs and exploring just why everything went down the tubes.
Links
- Horus Heresy (1993) at BoardGameGeek
- Horus Heresy (2010) at BoardGameGeek