The fall and rise of Athlanthean Empire

In the 41st millennium, the Athlanthean Empire was the greatest nation in the entire galaxy that stand for thousands of thousands of years. Led by a great emperor called "The Emperor of Mankind", millions of worlds were in control with great might and harmony. All needs are fulfilled and only a handful of poverty can be found in the entire nation. But that's all changed when a Son the emperor himself named Horus of make a deal with the chaos devils and rise to defeat the emperor. The civil war started and take billions of innocent live. Eventually Horus and the emperor face to face in a battle. The emperor manage to defeat his son but almost took his life in the process. Thus make the emperor weak and after several days he disappear. The impact of the civil war and the disappearance of the emperor was tremendous and led to the devolution of human kind. But after 10 century the emperor came back and ready to bring the Athlanthean Empire to its golden age. He promises to never leave the people of Athlanthean Empire alone again.