In Shingon Buddhist thought, a fierce deity is a benevolent being who will mercilessly slaughter any spirit or being that gets in the way of humanity's Enlightenment.
If we think about the game's overall theme, which is forgiveness and redemption, on top of the role as a hero that Link plays in the plot, this ties together everything beautifully.
Termina is a realm consumed by despair and hopelessness because of the Moon's slow plummet towards the Earth, which will destroy the world in three days.
Link, being the selfless person he is, in a way, through his actions, rejects the fear, despair, and hopelessness to save them all through sheer force of will. This ties in with all of the incarnations of Link being selfless, pure-hearted people who will do the right thing, and have unbreakable spirits. No matter how dark it gets, Link will succeed, come Hell or highwater.
Now, what's the description of the Fierce Deity's Mask in game by one of the Gossip Stones in the 3D remake?
The Fierce Deity's Mask, a mask that contains the merits of all masks, seems to be... somewhere in this world...
Eiji Aonuma went further on this when asked about what power the Fierce Deity's Mask contains:
The best I can give you is just a suggestion. The best way to think about it is that the memories of all the people of Termina are inside of the Fierce Deity Mask.
This ties in with the game's themes. Forgiveness, redemption, selflessness, and so on.
By helping out the denizens of Termina, you heal their sorrows, you help lovers reaffirm their vows of love to each other, you give a man the joys of freedom from his busy schedule, you remind a stressed mayor about what really matters, you help two ranch girls save their ranch from alien invaders and bandits stealing their milk, you heal the sorrows of a Goron hero who couldn't fulfill his duty, you give a Zora on the edge of death peace.
Your actions all throughout the plot are healing worries and sorrows and bringing them back on the path of enlightenment, and when you've helped everyone, Link, and you by extension, the player, achieve enlightenment by basically becoming a fierce, but benevolent god.
Why a warrior god, a fierce deity?
Since when is justice nice? Justice is righteous. Since when do the force of evil play nice? They don't. They require those who battle evil to be fierce and swift in enacting their judgment.
Everything about the Fierce Deity design plays into this as well.
For example, ever wonder why his sword a double-helix?
I've discussed it before in this forum, but the way the sword is constructed is like a pattern-welded Viking sword, but without the central core, which supports the Zen and Buddhist idea that everything rests on nothingness.
On top of this, deities in many religions are often depicted as serene and stoic.
Now remind yourself of Fierce Deity's look:
If you're still skeptical of the Buddhist themes in the game, remember that there's many ways to gain Enlightenment, according to Buddhists. For example, in the
long novel
Journey to the West, one of the main characters, Sun Wukong, the Monkey King, eventually achieves Buddhahood in Victorious Fighting, and if you accept my personal interpretation, Sun Wukong becomes a literal
god of victorious fighting.
It's not outside the realm of possibility that Fierce Deity is Link's own Enlightenment, becoming the pure form of stoicism, fighting skill, justice, truth, and selflessness.
But he is fierce because he needs to be. There's a being that gets in the way of the people's Liberty, their Enlightenment, their Pursuit of Happiness.
And that's the demon that possesses Majora's Mask.
Why do you think that the fight between Fierce Deity and Majora is so one sided?
Because the light of Enlightenment banishes evil spirits.
That's my personal interpretation of what the Fierce Deity is. It is a literal god of justice and truth that mercilessly slaughters evil spirits. Whether or not you accept it in your own personal headcanon is up to you.