Horus

Horus was the most favoured son of the Emperor.However after the virus bombing of a planet,Horus started a civil war called the Horus Heresy.The Horus Heresy brought the Imperium to its knees and it is only because the Emperor killed Horus that the Imperium survivied.Horus led a Legion called the Luna Wolves which were renamed Sons of Horus and now called Black Legion.