I don't like to relegate it to such obscurity. I theorize it has a connection to how the sages were even started. They received power from the gods to battle evil, namely the lingering power of Demise and the Demons. They truly expected that blade to kill the evil Ganondorf, who we know had powerfu; magic before he got the Triforce. What if that blade is from one of the god's themselves? These events occurred after Link came back to Hyrule as a child and warned Zelda. With the Master Sword still sealed in the Temple of Time, what if the gods of Hyrule called on new sages from a new realm. These brought with them a new power This power was very powerful (I can't believe I just said that), but it was also impure It was neither light nor shadow, it was Twighlight. That is why the sages were transformed into their mysterious forms; to survive in the light of our world. That is why the sword they used could be used by both evil and good. That is why the sages were near the Twighlight portal; they came from it, and they were going to return through it. The sword is essentially the Twilight's version of the Master Sword. The sages are the Twighlight realm's version of the sages, called by the gods to protect Hyrule.