Movie from my childhood name?

I once saw a movie where a master gives someone (his companion, follower, apprentice, etc.) a magical object and tells them to look after them until they're back. The companion (etc.) doesn't manage it, however, and an evil woman gets hold of the object and can use it to perform magic or something magical. She forces trolls and orgs to obey her. At one point, she even shrinks someone. What's the name of the movie?