An idea

Guys. Guys. I just had the best idea.

And by "best," I mean "stupidest."

Remember "Three's a Crowd"? Do you remember how, at the end, Discord gave Twilight a friendship medallion thing? She had, after all, gone to the ends of Equestria for him, and thus proven she was truly his friend . . . or at least that's the excuse he gave for his extended prank.

So what if at the end of "Twilight's Kingdom," that had been Twilight's key? Screw the fact that she never actually had an epiphany in that episode, and that there were no rainbows, and that it's highly unlikely that Discord actually was faking an illness as a test of Twilight's friendship. Turns out she had the key all along (except that it seems to have disappeared as soon as Discord stopped focusing on it). She just needed Discord to point that out.

Please don't hurt me, I didn't mean it.