If you've played Zelda and you're a hardcore fan, you may have heard of the Tetraforce theory. A theory that's supposed to state the existence of a fourth Triforce piece. There are many theories of what this piece means.
Of course these theories are not proven to be true but they could be possible as well as other theories of certain things in these games.
The theory I agree with, the most popular, being the theory that the Feirce Deity in Majora's Mask represents the fourth piece. First of all, the Fierce Deity is a god that destroyed Majora, a beast that represents the satan of the Terminian province. Majora wanted to destroy Termina, a possible alternate universe under the Lost Woods. Now back to the Feirce Deity. Apparently the Japanese refer to this god as 'Oni Link'. 'Oni' means Demon in Japan so it could be the reason that the middle piece of the Triforce is black which could also represent another theory. If that's not convincing enough look at the oni's clothing, it has an upside down Triforce piece. Just look up the Fierce Deity Link and look at his clothes. Also the Feirce Deity mask is given to you by the Moon Child wearing Majora's Mask if you collect all the masks. And the moon child says you have weak masks and he gives you the Feirce Deity mask, and says "Lets play good guys and bad guys" as you can see the Feirce Deity and Majora hate each other so they both faught to the death leaving only masks left as their remains. So both Link and Majora pretty much re-enact the battle once again but this time the Feirce Deity defeats Majora and saves Termina.
There are other theories but you can look others up as well X3
