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In Chapter III, Stage one, as you progress through the stage you'll find a dead Brotherhood Knight possessing a scroll that reads, "Last night I dreampt that a beautiful woman visited me in my dreams. She was bewitching, mesmerizing. She told me a warrior is coming who will destroy the Lycan Lord and that she and her people will help him. She was the Queen of the Fairies and she spoke in a language I could not recognize, yet I understood every word she said. I will surely never look upon such beauty again in all this world. If I die this day, I die having seen and experienced things no other man can lay claim to, and so I can happily declare that the 'Cake' is not a lie!"
The last sentence is a reference to the Valve game Portal.