This just popped into my head… Do Ghirahim and the Fierce Deity have anything to do with each other? Some people speculate Ghirahim is the embodiment of a fourth triforce/tetraforce piece and is sealed away after Skyward Sward. It’s also a theory that the Fierce Deity has to do with the fourth piece. Some evidence to this is the single triangle on the Deity’s breast plate. Perhaps Ghirahim was sealed away into the Fierce Deity mask? The two characters do have a similar color scheme and have tetraforce background theory… There are probably a lot of holes in this. It’s just a thought.
This is what’s keeping me up at night.