The Crystal ball is a key item in Dragon Quest VIII that the party is required to retrieve in order to progress the story. Kalderasha threw the ball away near the Waterfall Cave shortly after the death of Master Rylus. This eventually led Kalderasha falling into depression which motivated Valentina to ask the party to locate the ball.
The party eventually tracked the ball to the possession of Geyzer who gave up the ball after being defeated. Once returned to Kalderasha's possession, he would use it to illuminate the whereabouts of Dhoulmagus at the southern checkpoint for the party to pursue.