If you have released all the trapped souls in the seal stone along your way from the Mage Tower, you can get the last mission from the suspicious lamp on the way to Keturidu Grand Hall. Talk to the Seal Stone again.
Objective: Talk to the Sealed Stone
The Stone Whale that was protecting Hauberk woke up. Defeat the Stone Whale that is holding the soul.
Objective: Defeat Stone Whale
Return to Hauberk and release his last soul and wish for the future.