The crystal ball is a magical device used by the witch Morina to see the future.
"The witch stared into the crystal ball, deep in concentration. White clouds appeared inside. Glimpses of the future began to show themselves to her."
In Beyond the Kingdoms, Morina uses it to find out if the Masked Man's plans will be profitable for her. Pleased with what she sees in the crystal ball, she agrees to help him, and tells him where he can find the books he needs to start recruiting his army. It is also Morina's first glimpse of the Otherworld, and she begins to form an evil plan.
"While looking into her crystal ball she had seen much more than his future. Morina had discovered a world of opportunities for herself, and if she played her cards right, she could get much more out of this partnership than a kingdom."