This page explains how to start and complete the Misko, the Great Bandit side quest.
How to Start the Quest
Talk to Domidak and Prissen in the Dueling Peaks stable, near Ha Dahamar Shrine. Talk to them again and pay 100. This will begin the quest.
Where to find Misko's Treasure
Go south from the stable and cross the two bridges, then follow the small southward river to the end. Climb up to find cracked stones in the wall. Use a bomb to open it up and reveal the treasure.