I have a couple of ideas:
1: Make Link and Zelda change according to the Sacred Realm (Dark World) principle.
2: Make them change according to the Chaos World principle (that is, they turn into monsters).
3: Make them change according to the Mirror World principle (That is, they turn into whatever is the OPPOSITE of their hearts).
Or....
Make Link and Zelda change according to the Sacred Realm/Dark World principle until you get Moon Pearls for them, but after that... The Chaos World principle kicks in (implying that the Sacred Realm's properties override those of the Chaos), thus making them monsters until they gain a NEW item. Maybe call it the Chaos Rune?
