LAST EDITED ON 04-16-12 AT 10:34 AM (EST)Or if you head into Judaic doctrine, you might get the tester: the divine agent in place whose job is to push and prod on direct orders from the head office. But that's another trope entirely...
As for Luna, at this point (especially with the season finale next week), any lures which didn't work by this point won't benefit from much in the way of additional attempts. But (since you brought it up) as noted in that thread, the show managers went to war for her. She's a living incarnation of Dark Is Not Evil -- and various executives decided that was a hard concept to handle for a kids' show, and ultimately one they didn't want to touch. The periphery demographic not withstanding, there are unspoken rules for what gets presented to children in the name of screwing up the next generation just as badly as the current one. Those rules are the reason it's Princess Celestia and Luna -- Disney has apparently taught kids that Queens Are Evil. And those same rules say Dark Is Bad.
But the show's creators headed for the trenches -- and they got her on the screen as Dark Is About A Thousand Years Out Of Touch And Could Really Use A Hug. Which is all they wanted in the first place.
Not an idea you should have to fight a war over.
(It's probably worth noting that the show's creators have said Luna was influenced/possessed by an outside party during her turn. Asking what it takes to turn an alicorn is the sort of very scary question best reserved for Season #3.)
In Luna's case, dark also has No Indoor Voice.