Feirce Deity was not confirmed to be a god nor nice. None of those two facts are canon, just wanted. Here is my theory, but there is no proof or back up to it, just a theory like Phantasmagoric's. Feirce Deity was the bad guy because of a huge mistranslation problem, Feirce Deity is the Terminian Devil acording to Majora's Moon Child. Then, according to the Stone Tower/Babal theory, The Golden Goddesses created Majora who destroyed the anceint civilisation and seperated the once connected worlds of Termina and Hyrule. The Anceint Tribe was not run by King Ikana but by Feirce Deity, or as I call him Oni meaning Devil. Majora Killed Oni and took the Mask just as Link killed Majora and took the mask. I think that both Majora and Oni were msks, bt turned into hmanoid and returns to a mask when its work is done. In MM's final battle, Majora is a Mask, but turns into humanoid, then when given to the Mask Salesman, back to a mask. Same with Oni. By no means is Oni good, in my opinoin. The rainbow showed up when Link took off Oni, because the universe did not want Oni. Oni is bad. Majora is a corrupted Agent of the goddesses who grew off evilness when she was seperated from her creators and world. Again, this is a theory that likely can not be proven nor confirmed ever.
