I was trying to sort out relations between Dreaming (Lux), Undreaming and Storytime.
Concerning the latter, we know that it is the place where all stories begin and end. Dreams flow from Stark to Arcadia throught Storytime, being a force which shapes the universe. Everything started there with a Dream of Lux, also the twin worlds. Even division of them seem to not change the fact they are connected as they were made from the same Dream.
I was wondering how Undereaming came to being. Most probably, is started existing when also Lux did. Nevertheless, everything seems to be in order when Dreaming and the Undreaming are in Balance.
We know that at some point Lux started dying and it was because of losing his heart, which was stolen by Klacks. Therefore, Klacks may be the one who is to blame for the inbalance between Dreaming and Undereaming. Because Lux got weaker, the Undreaming became stronger and dangerous. I wonder if monks in Tibet would have been able to make their ritual if that balance had not got misplaced in the first spot. I guess probably not. Therefore, maybe Cortez's intention in the first place was not some evil plan but he wanted balance not only between worlds but also between Dreaming and Undreaming. If Lux stopped Dreaming, the world would cease existing. Therefore, Brian may have been sent to Arcadia to merge Dreaming and Undreaming back together but he failed as the latter took control over him.
There are some things which I trying to understand - why the Undreaming was residing in Storytime and the Dreaming in Arcadia and not for instance reverse? How Klacks manged to steal Lux's heart? We know we was powerful Necromancer but still Lux is a god. The opposite of Lux is Undreaming as they merged together at the end of Chapters but what about Yaga? What is the relation between the Sisters and the Undreaming? And why we were left with questions regarding Cortez role? This question should have been answered in DFC.