That was terrifying.
That was certainly something, alright. I'm not really sure where to begin with this one, except that it was fairly unsettling. If I were to compare this to another episode, I'd say this is reminiscent of "Lesson Zero". Here, Rarity is possessed by dark magic, and becomes increasingly deranged as the episode goes on.
This episode isn't the strongest in regards to character development. Rarity isn't herself for most of it, and Spike only changes around the end to learn his lesson. It's not a particularly complex episodes, especially compared to "For Whom the Sweetie Belle Tolls" or "Testing, Testing, 1, 2, 3". That said, Spike was written very, very well. His character is depicted well here. I always liked Spike, but this really is the best way to write him, as opposed to S3's "Spike at Your Service", for example.
This episode has a darkness to it that I quite like and which S4 had been missing. Dark magic is involved, and once it's in place Rarity almost immediately begins deteriorating. This dark magic is an interesting addition, and the episode's own mildly unsettling tone is definitely a breath of fresh air after the serious tonal stagnation that had occurred easier this season. As Rarity grows increasingly deranged, the episode's humour follows, becoming almost more worrying than funny. This is obviously intentional, as we're meant to feel concerned about Rarity's actions.
The humour is still in place, though. Rarity going on a comfort eating binge early on is particularly amusing, and comedic elements exist throughout. But these do give way to creepiness as the episode goes on. Unlike the "cringe comedy" episodes, these jokes don't make you cringe because they're awkward, but instead because it's a fairly funny event put in an unsettling context. As such, the episode is consecutively genuinely funny and more than a little uncomfortable, which makes for an interesting dynamic.
It's not a bad episode at all. It wasn't the funniest or most interesting episode, but it was funny and interesting enough, and that it's more than a little creepy does a lot to help it stand out. See you next week.