Far, far to the south, where east and west meet and become north, there is a valley, carven not by the slow slide of the world's rocky skin over her firey heart, but rather the wind and water have carven it, deep and narrow, folding away from the sun. Here, at its deepest depths, the Light never reaches. Here, at the top of the bottom of the world, lies the place which Shadow and Darkness have dedicated to themselves.
If a creature is transformed into another creature (say, a human into a dog), then that creature will become more and more accustomed to the new form until a point is reached where reversing the spell is impossible. A second transformation is possible, but the creature will have to go through the whole process again.