Realm of Chaos: The Lost and the Damned (1990), p176 — The Emperor and the Powers of Chaos

The Chaos Powers sensed the presence of the New Man and his efforts to curb their own power and growth. Even before they became fully conscious the Chaos Powers recognised the Emperor as their greatest enemy. Khorne was the first of the Great Powers to wake fully, and an era of wars and conflict raged across the globe. Tzeentch was next, and nations and politics grew to adulthood with all of their implicit intrigues and double-dealings. Nurgle was the third to awake and plagues swept across continents claiming many souls for the Lord of Decay. By the end of the Middle Ages all three of these Chaos Powers had awoken to full consciousness. The fourth Power, Slaanesh, still slumbered.