The Horus Heresy is reckoned by many to rate as the greatest single disaster ever suffered by the Imperium. The specific details of the heresy are known only to the Emperor, but its broader history is the stuff of popular legend. According to one version of the tale, Horus was once the most trusted servant of the Emperor. But in his heart there dwelt a hidden evil, and he became seduced by this evil, and came to nuture demons and other forces of destruction. Horus marched upon Earth with a third part of the hosts of the Imperium which he had seduced to his purpose. For seven days and seven nights the hosts battled until the Emperor caught Horus by the heel and cast him into the Eye of Terror and with him the third part of the hosts of the Imperium.
-Book of the Astronomican
The story of the Heresy has changed over time, passed from generation
to generation. He is the story in its full and complete form.
"...but in the Warmaster's heart there dwelt a hidden evil and he became seduced by this evil and came to nurture Daemons and other forces of darkness. And he did march upon Terra and with him marched a third part of the hosts of the Imperium which he had seduced to his purpose. And there was terror and bloodshed and for seven days and seven nights the hosts did battle, until the Emperor - bless His honoured name! - caught Horus by the heel and cast him to that place they call the Eye of Terror and with him the third part of the hosts of the Imperium..." Taken from Codex: Chaos Space Marines v3