The quest to free the Masamune of the curse and acquire the Mastermune involves four steps, and can be done at the same time as the six dragon quests. It is easier to do if you have fought the black dragon and stolen the black plate from him.
Take Riddel back to Home Forbidden island, where you will find out the rest of what happened when Karsh and Dario went to Isle of the Damned. After that, you must fight Dario. Dario has very strong black skills, which can instantly kill Riddel -- if you stole the black plate from the black dragon, put it on Riddel before this fight!
Dario will counter any elemental attack with an elemental of opposite colour. White and black attacks are countered with very strong elemental attacks, other elemental attacks are countered with status elements. I do not recommend using white or black attack elements unless the character using them has equipped the plate of opposite colour. This one's hard, good luck!
If you survive the battle, you will receive Riddel's level 7 technique, as well as the Mastermune. Dario will go to Home Viper Manor where you can find him repairing the manor toward the end of the game.