The ground becomes rough and hazardous, the real floor superimposed with the irregular terrain of the Plane of Shadow.

You cloak the ground with an uneven and hard to traverse surface. The terrain becomes difficult, meaning that each 5-foot square within the area costs double to move into. For instance, each light undergrowth square (normally costing 2 squares of movement to move into) now costs 4 squares of movement to move into. If you cast this mystery a second time on the same area (or a portion of the same area) while the first casting is still active, the second casting does not worsen the terrain further (although it would extend the duration of the effect on that area).