Horus Heresy

It was pretty much this.

The biggest single event in the Warhammer 40,000 fluff, this is when the Space Marine primarchs rebelled against the Emprah under the banner of Horus. The battle messed up Holy Terra and then something terrible happened! The Emprah was mortally wounded and now has to sit on the great golden bog for life-support, and the traitor SPESS MEHREENS retreated into the Eye of Terror where they mutated into the Chaos Space Marines.

Warhammer 40,000 Fluff

The Horus Heresy is the most unfortunate event in the lore of Warhammer 40k, because it is really what makes the year 40,000 and the ten-thousand years before it so damn bleak and depressing. Prior to the Heresy (read: year 29,000), everything was bright and shiny and more or less happy.. comparative to the current game year. But this is not blasphemy or madness, THIS IS GRIMDARK! Haha! Disregard that, cannon says things were still Grimdark, what with planets and systems being scoured, partial lobotomy to make people killing machines, Chaos Gods, etc, etc.

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.

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)

Links

• Horus Heresy (1993) at BoardGameGeek
• Horus Heresy (2010) at BoardGameGeek