3. Lord Of The Rings: The Fellowship Of The Ring
This series is based around the ring of power. It is an evil ring with much power and the evil Sauron is trying to get hold of it. Yet despite this, It falls into the hands of Bilbo Baggins who then gives it to Frodo who is given the task of destroying it. Wonderfully, this ring can cause the wearer to become invisible which saved both Frodo and Bilbo at the right moment without them knowing that it would. At the same time it also will allow the Eye to see where they are which alerts the Ring Wraiths of their location. By wearing the ring, it both saves the carrier and allows them to be in danger. It allows the story to progress with imminent threats instead of the antagonists to wander aimlessly looking for someone with a ring. It both saves and destroys them, making it one convenient plot device.