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.
An ancient, psuedo-immortal race has been manipulating something important (politics, religion, economics, etc.) for centuries. Hardly anyone is aware of them, let alone their goals.
It is traditional for two warlike countries to each give their first-born royal prince as a hostage to each other to prevent war. One of the princes has been murdered, and the PCs have a few days to rescue the prince in the other country to save him from being executed and a bloodthirsty war from breaking out.