The Emperor shaped all that you see, all that you shall ever see, for such was his will. And such was his will that man should walk upon the land, so we did, and we were happy, and the world was a paradise.
But Horus-Moon, who was the Emperor's favourite warrior, looked down upon the Paradise-of-Man, and grew jealous, for his was the sky, and never the ground, which was perfect. And so Horus-Moon grew to hate the Emperor, for Horus-Moon was a spirit of pettiness and vainglory, and could not stand the Glory-In-His-Name of the paradise below.
And so it passed that one day, when The Emperor's back was turned in the act of creation, Horus-Moon declared himself Master-of-War, and stole Sun-Throne from The Emperor.
Horus-Moon, however, was far from the warrior that The Emperor was, and he could not fight the evil which The Emperor, in his wisdom, had sealed away within Sun-Throne. Horus-Moon unleashed the evil into Paradise-of-Man, and that evil was the Chaos-Heat, and when it approached the land turned to sea and the sea cried out with the dying breaths of a thousand drowning men.
But The Emperor heard Man cry for help, and He heard the laughter of Horus-Moon, and he did rush to do battle with his wayward son. Blows were exchanged and injuries were grave, for though The Emperor was a warrior without peer, Horus-Moon had absorbed much of the evil of the Chaos-Heat, and he fought without honour or virtue.
For many centuries Horus-Moon and The Emperor fought, and though they battled in distant lands, Man could feel the struggle. So it was that First-Despair came to Once-Paradise, for Man could not aid the Emperor from so far away, and though they had lost paradise, this was their greater shame.