Turns out I never fucking posted the "Fierce Deity Link is Link's buddha mode" rant. So here goes, modified from the messages I sent to my little brother over a year ago, otherwise I can't get peace of mind.
In the game, Link actually becomes a buddha, and the Fierce Deity form is when he reaches that state/unleashes that aspect of himself. Fierce Deity isn't just a sick ass name for the form, it's literally a type of deity mostly associated with Tibetan Buddhism, if I'm not mistaken. Fierce deities, also known as wrathful deities, are basically aspects of buddhas that protect others from threats to enlightenment (Majora, amirite?) and clear the path for that in a spiritual sense.
How does Link acquire the Fierce Deity mask? First he has to help everyone in Termina to get their masks, THEN he has to give all of those masks, his possessions, away. Off the cuff, I'd say giving up your earthly possessions is one of the bigger tenets of Buddhism as a whole.
And now let's take a nice look at Fierce Deity's design. If a character's design is good, every part of it has a purpose.
What is the dominating color of the design? That's right, it's blue. You shouldn't be surprised to know, then, that blue in Buddhism symbolises wisdom, purity, and healing.
Fierce Deity has a dark blue bodysuit underneath. Well, in Hinduism (which is honestly kiiiiiiiiind of adjacent to Buddhism) blue skin is basically a sign of a heavenly, enlightened being. And that bodysuit Fierce Deity is wearing sure is tight enough to look like skin.
His eyes are white. In a lot of cultures this usually symbolizes possession, but in Buddhism specifically the Buddha is often depicted with his eyes only partially open, which could leave only the whites of the eyes visible. Similarly, his hair is white (I'd argue silver but same thing in this case), which is the color of purity. Also usually in pop culture, especially in Japanese media, silver hair is the mark of an exceptional and often a supernatural character.
As for the patterns on his face, all I can think of is kabuki makeup, kumadori. I haven't found any evidence that makeup like that is used in Buddhism. In kumadori, red color is used for heroic characters, while blue is used for villains. Why Fierce Deity has a blue pattern despite being a hero, I have no clue. Red and blue colors can be used together in one pattern, but the blue always carries a negative connotation. But moving on, in the official illustration Fierce Deity seems to have red patterns in the place of eyebrows, but in game they're brown despite him having white/silver hair. Well, in kumadori brown makeup represents monsters and non-human spirits, which is kinda fitting in this case.
Moving on, Fierce Deity has a crescent moon on his armor. I'm not entirely sure if it was made into a crescent moon only to make it clear that it's a moon symbol, or if the crescent itself represents something. But the moon in Buddhism symbolises truth and enlightenment according to some source I managed to dig up once and then lost. It could also represent the mind in itself. The moon does have some meanings deeper than I can get to, but it's also important in denoting significant days for Buddhism in the lunar calendar.
The triangle symbol is also pretty vague. Legend of Zelda has the Triforce, but this is only one triangle, not all three, so it might have another meaning. In Buddhism, according to some sources I could find, triangles symbolise the "three bodies of buddhahood" which are the bodies of ultimate reality, bliss, and the physical body we all have.
Then let's look at the three gems Fierce Deity has on his neck. It's not just some meaningless bling to tie the design together, because they do have a meaning. As a set of three, they likely symbolize the Three Jewels of Buddhism, where each stone represents a part of buddhahood: the Buddha, the dharma, and the sangha. These, in layman's terms, mean "the exemplar", "the teachings", and "the religious community" respectively. So I suppose that in Fierce Deity Link's case it means that he takes part in all of these aspects and thus is enlightened.
Lastly, the sword. It could be an extremely stylized vajra, a kind of a pointed battle club that is an indestructible and irresistible force. There's a lot more symbolism behind the vajra than just that, though. The sword could also be based on something called the "infinite knot" which overlaps itself without beginning or end and symbolises infinite wisdom and compassion among other things. The sword has one blue and one green blade. Blue is, again, wisdom. Green, on the other hand, means peace and balance.
Also, the line "could this mask's powers be worse than Majora's?" when receiving the Fierce Deity mask is pretty clear when you know what kind of being he is. Fierce deities deliberately make themselves appear demonic and terrifying to scare away demons. He's also meant to be op, not just as a treat for the player for getting all the masks, but because that's what fierce deities are. Also it's rich coming from the literal villain of the game who dies in the next 3 minutes thanks to Fierce Deity.
All in all, 11/10 design. To make it any clearer that this is Link as a buddha you'd have to slap a lotus flower or a dharma wheel on that bad boi