Or as EQD put it, 'King Sombra x Stairs OTP'.
In other news, being un-discorded apparently changes your hairstyle -- it's not just Pinkie Pie! And Cadance is apparently the head of the crystal empire, which is protected by a periodic party where as long as everypony smiles, evil is kept at bay. My crack theory that Pinkie Pie and Cadance are the same pony is not exactly contradicted by this. @.@
"I want a flugelhorn." "Anypony else want a flugelhorn?" "IWANNAFLUGELHORN!"
The plot of the episode wasn't too awful; they figure out how to save the empire by interviewing everypony and doing research in the library ("I'm not sure I actually work here."), then the mane cast keep everypony happy while Twilight fetches the magic crystal thingie. The villain didn't get much characterization, but the whole infectious darkness thing was kind of spooky.
Shining Armor is still an idiot, though. He's a *shield* pony so he fights the villain who has no physical form but is easily stopped by barriers by using... energy beams! And fails, and gets his magic sealed, and then we find out he was leaving Cadance to put up the shield around the empire anyway...
Although to be fair it looked like KS had really strong anti-magic so Cadance's positive energy shield might have really been the only thing that would have worked anyway. Still, the only useful thing he did was give moral support to Cadance. They totally need an episode where he gets kidnapped by a dragon or imprisoned in a tower or something.
Anyway, I liked it, for the most part. The part I mostly didn't like was Twilight's Test song and the reprise; the crystal pony song was a lot less stupid in context but that one was still just kind of 'meh'.
Further thoughts --
(1) King Sombra was probably not actually all there. He acted like a mindless shadow of his former self, and there would be a very good reason for that to be all he was.
(a) ...which means Celestia's decision to leave it all up to Twilight wasn't as risky as it sounds. She probably could have swooped in and saved the day at any point, since her *best spell* is dispel magic and she demonstrated that it worked on evil antimagic crystals.
(2) I bet Twilight failed the test as Celestia intended to give it, but got a pass anyway. That, or the real test was 'how do you handle authority' in general and there was no specific plan to put her in the no-win situation.
(b) ...that, or Celestia is actually a freaking oracle.