After Kazus, your next stop should be Tozus, a hidden village accessible only by shrinking your party with the Mini spell. You'll need to visit the Elder of Tozus to learn this spell. Once shrunk, you can enter the village and interact with its tiny inhabitants. Tozus offers some unique equipment and spells, so explore thoroughly. Your main goal here is to find the Viking's Cove, which is also only accessible in Mini form. The Viking's Cove is a small dungeon that leads to the Viking's Hideout. Be cautious, as enemies in Mini form can be surprisingly dangerous, and your physical attacks are significantly weakened. Rely more on magic in this state.
Inside the Viking's Hideout, you'll meet the Vikings, who are being tormented by a monster. They will ask for your help. This leads to the next dungeon, the Nepto Temple. Ensure you have a good supply of healing items and MP for your mages before proceeding. The Mini status will be a recurring mechanic, so get used to its implications on combat and exploration.