Have you ever read the Majora's Mask manga? There was one.

At the very end it had a bonus story. I can't really think of it all off the top of my head, but there was a hero, or traveller, or something. And this demon. Apparently, he ate people. The ones that wanted his armor, scale, thingies. Something like that.

Then they talked and blah.

Something happens.

And the guy plays a song, I believe. On a harp. The monster dances and dances and eventually dies. Then the man, who defeated the monster, took off one of the monster's armor things and made the Majora's Mask.

Sorry for any inaccuracy. I'm sure I'm close to the story.

But that's where the mask comes from. I'm sure that fits into your story somehow.

I'm really impressed you found out so much, though.