We flash back to Filly!Twilight asking Celestia why Equestria is so "disorganized", with ponies largely doing whatever they feel like, when surely the Princess could arrange things more efficiently. Celestia reminds her of the difference between a monarch and a tyrant, leading Twilight to figure out herself that that's just not how Celestia, or any good ruler, rolls.
Cut to present Twilight raging at her staff and remaining threats over the theft of the Elements- even though she now feels so soaked in power that she doesn't need the Elements for anything beyond her wounded pride. With pretty much no more triggering, as her colors fully shift to that of a Nightmare, she somehow channels both Voldemort and Galadriel and proceeds into full-on villainous monologuing about taking over Equestria so that Order will reign supreme...
...and then Fluttershy, having had quite enough of this, flies up and lectures and Stares the Nightmare into submission.
Honestly, it's a lot more awesome than it sounds.
As the other Princesses arrive, Twilight awakes in possession of herself, but she can no longer cast anything except dark magic, and Luna reveals that she will have to be Purged, a holdover from the days when unicorns were far more powerful and moved the celestial bodies themselves (and often delved too deeply into magics they shouldn't). Another unicorn of comparable power, and with no strong emotional significance to the first, must merge his or her magic with that of the first to draw the victim back into the light. Celestia had tried it back before banishing Nightmare Moon, but there was too much emotional baggage for the dark magic to take advantage of, and Luna was not particularly interested in being saved- unlike Twilight.
And it just so happens that "the Countess Mercedes" has arrived, ready and willing to serve as Twilight's Firelighter, and to escort her (with only Spike's additional company) to a remote location where the ritual can be carried out in safety.
Believe it or not, this is supposedly the end of the darkest part of the story. And why not? Twilight has at least started on the path to reconciliation... just leaving a would-be usurper or two crouching for the kill....
