Maddoc sat amid a pile of unrecognizable corpses. Men, Dwarves, Orcs, you couldn't tell. He periodically took a stab at one with his new knife and screamed, "STOP LAUGHING AT ME!"
I am the mask that grins and lies
I'll hide your face and shield your eyes....
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.