Awakening of Evil

I've gone through the entire forum, but I still don't understand the labyrinth. I'm still interested in exploring everything. In mission 2, I explored the entire labyrinth (thanks to the infinite corridor). I've read all the books, and the quest mentioned a dark book and some dark rune. There's neither in the labyrinth. After exiting the labyrinth, the quest remains. I'll continue playing, hoping the rune and book are on the surface. But I spent half an hour searching the labyrinth, even checking all the treasure chests in the hope that the rune was there. A hint would be helpful.