Misko, the Great Bandit

"Misko, the Great Bandit" is a side quest you can find in the Dueling Peaks region.

Game progress required: Reach the Dueling Peaks Stable.

Starting the quest: Go inside of the Dueling Peaks Stable and talk to the twins Domidak and Prissen standing near the back of the stable. You will overhear them discussing some secret. Talk to them again and they'll offer to let you in on it, but they ask you to pay 100 Rupees to hear the details. This will start the quest.

Guide:

After paying the twins 100 Rupees, they will give you this lead on some hidden treasure left by the great bandit Misko: "The little twin steps over the little river. My cave rests above that river's source."

If you look on your map to the south of the stable, there is a bridge named "Little Twin Bridge". This is the bridge mentioned in the clue, and the river is the same as the one running underneath that bridge. If you follow that river roughly east to where it branches off into a smaller stream, you will find the "river's source". Climb up to the cracked rocks above you and bomb them to open up the cave. There will be several chests waiting inside. Take your loot and the quest will be complete!

Reward: This quest rewards itself with the treasure you find in the cave. You can go talk to the twins again and tell them you found it, but this isn't required to finish the quest and they won't give you a reward themselves.