Background of the 41st millennium

It's the 41st millennium.
Mankind is in another Dark Age.
Civil War broke out within the Imperium of Man.
What was once brothers were no more.
All of the men in the Adeptus Astartes were in question.
Horus, the Emperors trusted son, was the first.
Many of the legions of the Astartes followed.
Chanting their war cry's.
"For Horus!" "We serve the Warmaster!"
Of those legions who followed Horus.
Sons of Horus
Word Bearers
World Eaters
Death Guard
Iron Warriors
Thousand Sons
Emperors Children
Night Lords
Alpha Legion
All those legions followed in the ways of the dark.
 
Those who are still loyal to the Imperium will never forget their names.
They will never forget what they have done.
Some on both sides of the war traded sides.
Loyalists turned to Heretics.
Heretics turned Loyal.
No one knew who could be trusted.
 
The end of the Heresy era,
Horus was slain after dealing a mortal blow.
The Emperor fell, only to leave his physical body.
Thus we entered into the Dark Age once more.