here's my theory
there are 2 main villains in the Zelda franchize, ganon, and vaati
now, ganon is just a reincarnation of demise
what if vaati was a reincarnation of Ghirahim. they look similar.....ish
maybe vaati revives ganon, then the somehow uses princess Zelda to turn ganon back into demise, then demise uses his demonic powers to turn vaati back into ghirahim.
the master sword would detect this and fi would be released
course its just a theory
A GAME THEORY!!!!!!!!