The Moon Mourners (I'll just sum this up to prevent boredom) were once part of the Alpha Legion. Just before the Alpha Legion switched sides, they formed their own band of warriors. Although small, they were very well trained in Guerrilla tactics. Soon after the Alpha Legion turned, they began to feel the warp overtake them. After the death of Horus, they turned to Chaos falling prey to the malevalence that is the warp. Soon, they realized their error and tried to return to the Imperium.

The Imperium essentialy said " ?! You expect us to take you back after fighting with chaos? no way!" and then they tried to kill them. Why'd they try to kill them. Mutations mostly. Soon after being chased left and right, they stopped running and killed their pursuers remembering the teachings of Alpharius. they havce become misanthropes and mockery of their former glory hating both the Imperium and the very stuff of Chaos and that which they have become.

How is it?