This is the quote that starts most Warhammer 40,000 books (Such as; There is Only War, Gaunt’s Ghosts series, etc.). This one sentence fits the universe of the entire Warhammer 40,000 extremely well. Humanity has fallen from it’s golden age, the xenos follow their own paths for their own survival, and Chaos conspires to throw everything into chaos (of course).
Abnett, D. (1999) Last and Only. Nottingham: Black Library