Planes of existence in my story...help needed?

Something evil, dark or cynical. You have a library to balance all the planes, but they are all sorta alike. You need something to balance all the good. Also, if the fourth plane is evil, then you have something to create a plot in, such as "fourth plane invades library, and one person from each other plane must band together to stop the invasion" or "outside source threatens balance and the four planes must work together and learn to trust each other"