ooooh, my turn! The fierce deity mask has alot of mystery behind it, but with the stuff::
The fierce deity mask can only be obtained by giving all 23 masks to the four playing children (which unchangingly resemble the bosses of the game, ill explain this at the end *). When that is done and you have played their games (which is actually really quite hard) you may talk to the fifth and alone child.
This fifth child has alot of mystery behind him, he obviously is alone and angry. His subconscious (likely the one of that ‘puppet’) is tormented by loneliness and is very angry. The mask, fuelled by this, made these realities true, creating the four other children (the bosses of the game) and bent on suffering. He just wanted to play with someone. But with the coming of link and him saving the giants, this aggravates ‘majora’ even more so he sends the moon down faster, which fails. When the kid sees link, he views link as evil and vile. So he creates the fierce deity mask for him. But wait, the masks resemble the dead souls of the past (deku, zora, goron, they all died before u got their masks), and it appears to be link. In Japanese, the fierce deity link is called the Oni link mask, meaning “demon link mask”. This adds a whole genre behind who link really is. Was he a demon before he become the hero of time? Did the gods destroy this evil soul and imprisoned him to become the hero of time for eternity. Is this why link is practically 100% good, cause he is a slave, rid of his evil?
But that’s another story.
When you get the mask, you can only use it in boss battles (the mask is a illusion of majora, therefore it can only be used with his other ‘illusions [bosses]), which is a shame, but its hell fun! You get two huge twin blades, which are like 4 times more powerful than the kokori sword, and they glow blue at times as well. Your health is also better, resulting in the final boss battle being solved with only a few swipes. But hell! Its fun and definitely worth the effort, you can do this to the other bosses as well. There is a glitch that does allow you to use it out of boss areas, but I don’t know how to do it.
*Back to the deep stuff, the playing kids do not want to play with majora cause, despite his power, he resembles nothing, so he is useless. This angers majora so he kills the children and created illusions (bosses) out of their newly formed masks. This explains why you kill the bosses, you get a mask. And you can replay the boss by recreating that illusion. When you recover the masks, you save the guardians, but also revive the children. This really does anger majora. The children ask you for your masks because they want to be someone else and play different games later. So you collect the boss masks for someone else, so that’s why you cant wear them.
Whew, what a big explanation, hope you understand jiarachi. Im happy to answer more questions if you have anymore.
EDIT, oh and i got the mask and it ROCKS, you should try get it, its fun!