You read/see that it is a book about how an ancient, evil being called Arik tries to escape from the dimension where he was trapped.
First, he sends out one of his Eyes, which looks like a ruby. It allows him to channel some of his power past the dimensional barriers and call his followers to it to perform a ceremony to free him. The ruby kills the crops, brings disease, and draws chaotic monsters such as koblds, goblins, etc to cause confusion. It also turns the immediate people around the ruby to stone and creates a magical force field to prevent anyone from stopping the ceremony.
There are three ways to destroy an Eye: Smash it with the Sword of Arik, which is used in the summoning ceremony. Have the white dragon Ariksbane breath on the ruby. Or play three particular notes on the Ice Harp, which will shatter the Eye.