Heck, I've even had the story shifted out from under me while in complete control of the story line, so I REALLY don't do more than have some semblance of an idea which can morph at any time. 

At present, the only driving idea I have is that this area is far more magical than it ought to be and that Wren has been under the magic's wandering eye for some while. In some way or another, the ancient magic of the area has wanted Wren and if that is to fulfill Chall's destiny or to help the area out or make a change in the country or... whatever - there is an intelligence of sorts which is keenly interested in the pair of them. :)