Hello,

I remember talking through this puzzle on my forums a while back. We didn't have many more ideas than you do. There isn't much evidence to help us.

It might help to delve more into masks. In MM, some masks have magical powers. A mask can either contain magic power (as in the Re-dead mask) or it can be the incarnation of a spirit (like the Zora mask). The masks themselves are created with magic. In the case of MM, Link creates several masks with the Song of Healing.

In the case of Majora's Mask, I think the mask is a combination of the two kinds of masks. The mask was created for use in hexing rituals. Evil magic must have been used to make it. (I don't remember the exact story, so I might be slightly off here). After the mask kept being used, it got more and more powerful, until the tribe became scared of it. I think that the mask gained so much power from its evil use that it was able to became an (almost) living incarnation of evil. It would begin to control and manipulate whoever wore it. The tribe realized that, put into the wrong hands, the mask could use somebody powerful in magic to destroy the world. This is why they hid it away, hopefully for all time.

Why the mask vendor wanted the mask, and how he knew about Termina, is a mystery. He seems to have some magical power himself, but good magic. Majora's Mask was a temptation for him, and attempting to get it at all was foolishness. But he seems to have wise reasons of his own, if not to just complete his mask collection.

When Skull Kid stole the mask and put it on, the mask took over. As weak as Skull Kid was magically, the mask was powerful enough to try to destroy the world through him. When that failed (Link summoned the four giants), the mask had already grown powerful enough to ditch Skull Kid altogether and retreat to the moon.

The "moon" scene is the strangest bit of the business. Somehow I think it represents the good power of the mask vendor striving with the evil power of the Mask. Without a user, the mask's evil is weak. But Link entered the Mask and challenged the evil power and its four evil cohorts. As Link attacked the mask, it was forced to reveal more and more of its power.

The Fierce Deity Mask must either be the mask vendor's contribution against Majora's Mask, or the double of Majora's Mask (kind of like yin and yang. Majora is the dark side, Oni is the light side). Why the Oni Mask would pose a peril to its user is also a mystery. Perhaps because its power is derived from Majora's Mask.