After you get the Eternal Sword, you can reveal Dhaos's hiding place.
Go to Ary, which is south and slightly west of Alvanista. Go into the inn and pay for the night. There will be a long cut scene. When you leave your room, someone will come talk to you.
You will get the Mana Earring, which you should equip on Mint.
You need to go to Dhaos's castle if you want to do the last remaining side-quests, so go to the cave north and west from Ary.
Go up the stairs. Follow the path to start a cut scene.
Go south to be transported back to the mountain near Ary.