First Catcan War

"And in that time the Great Beasts shall walk the earth, beneath their tread shall cities become as dust by their burning breath shall civilisations be as ash, So the End of Days is begun."

- The Apocrypha Terra

The Horus Heresy was a galaxy-spanning civil war that consumed the worlds of Mankind for 7 Terran years. Its outbreak marked the end of the Emperor of Mankind's Great Crusade to reunite the scattered colony worlds of humanity under a single government and the beginning of the current Age of the Imperium. The Horus Heresy is in many ways the founding event of the Imperium of Man as it now exists. The conflict was fought across the Milky Way Galaxy in the early centuries of the 31st Millennium and resulted in more than 2.3 trillion dead, 4.6 trillion if one includes the planetary populations purged by the Imperium after the Heresy due to the taint of Chaotic corruption. The Heresy concluded with the death of the traitorous Warmaster Horus, the internment of the Emperor in the Golden Throne and the creation of the Chaos Space Marine Traitor Legions. The Heresy was directly responsible for the birth of the present-day structure of the Space Marine Chapters following the Second Founding and the Reformation of the Imperium by the Ultramarines' Primarch Roboute Guilliman.