An entire mare-a-thon of all eight seasons of Charmed (which I hadn't seen before but I heard about thanks to Duman from Winx Club being inspired by Dumain) all led up to this moment, the episode of Starlight's redemption and possible Chrysalis and Sombra return.
I've said it before, probably my second favorite episode next to Twilight's Kingdom, but once I rewatch it I might decide whether it should even go above it. Thus being the second episode (and second season finale in a row) to overtake A Canterlot Wedding.
Going into it, I thought there would be one alternate timeline where all the villains were in power, either allied or slugging it out for the throne. So, I wasn't expecting Starlight to stop the Sonic Rainboom that early in. Upon return to the future, King Sombra is in power, and Celestia is leading an almost Lord of the Rings style war against him. Sadly, he gets no lines, but he does have some screentime, and Rainbow Dash, Pinkie Pie, and Maud are fighting against his army. So now we know what would have happened if he had had a chance to do more - he'd try to take over all Equestria.
So then Twilight tries to stop Starlight from altering time, but Starlight convinces the jocks not to pick on Fluttershy, preventing the race from even happening. And this leads to a second alternate timeline - this time with Queen Chrysalis in power. Like in the comics, Fluttershy is the first pony to ever mention her name in the show. So now Twilight knows the changeling queen has a name. We get to see Chrysalis transform again - this time as Applejack, so that's what it looks like when she turns into somepony that isn't an alicorn. Oh, and Zecora gets the spotlight when she's the one leading the war against Chrysalis in the Everfree Forest.
After that Twilight tries to fight Starlight, but Rainbow Dash is distracted by this, leading to a perpetual night timeline run by Nightmare Moon, where Rainbow Dash and Rarity are working for her. The torches of blue flames add the perfect atmosphere to this and reminded me of the Moon Temple in Star Fox Adventures. Nightmare Moon imprisoned Celestia in the moon and tries to force Twilight to send her back in time, but Twilight tricks her into defeating the timberwolves (which of course would reassemble in moments) then once again overpowers her with teleportation and gets through the time portal with Spike.
Then after that we get a brief glimpse of more altered timelines as Twilight continues to unsuccessfully try to stop Starlight: First we get Tirek destroying Equestria, then Discord with his eternal chaos and chasing Celestia and Luna around as clowns, and then, quite randomly, the Flim Flam brothers tearing things down with bulldozers.
So finally Twilight takes Starlight along with her to a desolate wasteland future, which I'm guessing is a result of Nightmare Moon, Discord, Chrysalis, Sombra, and Tirek all coming to power at once and fighting each other over it, possibly destroying themselves along with all of Equestria. Then Starlight shows Twilight her tragic backstory revealing why she hates cutie marks so much. As she is about to tear the scroll in two, I thought the episode would either end there in cliffhanger or there would be a permanent timeline change. But Twilight convices Starlight to give friendship a second chance, and we get our long-awaited Starlight redemption. Rainbow Dash does the Sonic Rainboom, restoring the timeline, and the scroll disappears forever.
Starlight appears to become one of the Mane 6, which I guess is now the Mane 7, which will definitely be something to look forward to in Season 6, which I don't think will get stale now that we have so many possibilities of character development for her and her going along for the ride on all the "Mane 6" adventures, including the fight with whoever the Season 5 villain is unless Starlight was grand finale for that.
I was expecting a lot of rage for Starlight's redemption, but most users here are able to accept it and like her more due to her backstory.
One minor thing I noticed is that as a filly her mane is in the same style as Aria Blaze, but I do not think she is her - just the same coloring scheme, like Derpy and Dinky Doo who aren't even sisters. Besides, Coloratura certainly can't be Sonata, being an old friend of Applejack as a filly when Sonata was banished as a siren a thousand years ago.
Of course, aside from Starlight's redemption, the biggest highlight of the episode was the different alternate timelines Starlight created by butterfly effect (which is even foreshadowed by her sitting on Fluttershy's throne), and how it gives us a chance to see Chrysalis and Sombra again. Nightmare Moon's timeline being just as focused on was probably because she was the first villain of the show even though her first fight with Celestia was already shown, and let's face it, Tirek and Discord were effectively in power already in their episodes, Tirek just needed to get the Alicorn Magic to become unstoppable.
So, here's a poll on the alternate futures: (excluding the desolate wasteland one)