Head west after exiting the tunnel, you will see a cave where all the vikings reside.
Enter the cove and make sure to collect all chests from hidden areas. Then talk to the captain on the map to a trigger a small scene.
Next, head to the exit to get back to the world.
Chests (6):
- Spark Dagger
- Thundara
- Fire
- Blizzard
- Viking Axe
- 3000 Gil