Dear Princess Twilight Sparkle,
I've realized neither of us were right.
-Your Student, Starlight Glimmer

That was what the letter in Starlight Glimmer's room said when Spike found it early in the morning.

At first, Princess Twilight thought that finding Starlight Glimmer would be hard, she had evaded capture for a year before. Instead, it was like Twilight was being led on a trail of breadcrumbs. Twilight for a moment wondered if this was a trap... but it didn't feel right. There were far less elaborate ways to isolate Twilight from her friends and get her to a remote location.

It led to, of course, the Everfree Forest. More than that. It led straight to the Castle Of The Two Sisters.

She found Glimmer inside the main hall. She was in the middle of a rune engraved into nearly the entire floor, with others engraved into the walls. They were so complex that Twilight knew there were maybe half a dozen ponies in Equestria alive who would be able to understand them.

"Hello Twilight," Glimmer said, not looking surprised, as she did some final adjustment and checking and rechecking her work. "I'll admit I was being selfish in my letter. I knew you'd look for me. I just had to show off my work." She gave an ironic chuckle. "Not very equal of me is it? ... Didn't bring the others? ... I guess that's for the best."

"Starlight... " Twilight hadn't figured out what the spell was yet, but she recognized the individual components plain as day. "Starlight no. Please no. I thought you understood! I thought you finally understood how vital to living free will is!"

"I'm doing this BECAUSE you taught me how important free will is!" Glimmer shouted.

"What?..." Twilight's jaw dropped.

"I knew my equal marks smothered free will... if I could have a village full of friends with no differences to drive them apart, I thought it was worth it. Then you destroyed it... and all I had left was revenge... and that revenge ended up destroying the world. So I had nothing left. You gave me something when you had every right to leave me rotting in a dungeon till the end of my days. And for that, you have my eternal gratitude."

Twilight Sparkle pleaded to her friend. "Starlight! Just because friends are different doesn't mean those friendship are going to end! You can't keep living in that one moment where Sunburst left you!"

Starlight stamped her hoof. "This isn't about my egocentric abandonment issues! Princess Twilight! Do you remember what we talked about last week?"

"... I told you about the other timelines I saw."

"And remember what I asked?"

"You asked... about the cutie marks my friends had in those worlds."

"Exactly. And what did you tell me?"

"... That they were all the same."

"Exactly... every timeline, no matter how much things changed, no matter what roles they had, no matter how different their lives were... the same cutie mark, over and over, and over again. IN SPITE OF THE FACT that they NEVER GOT their cutie marks on that day of the Sonic Rainboom! If events had continued on IN SPITE OF my efforts, I could accept it... EXCEPT! The entire POINT was how my meddling was spiraling reality further and further beyond recognition! Our destinies are what we MAKE IT Twilight Sparkle! Otherwise we're just actors playing roles! When you told me about when you scrambled your friends cutie marks, I figured it was because they were cutie marks they hadn't actually chosen themselves that they were unhappy, lacking the actual skills and abilities they thought they were supposed to have. Instead...

"You've taught me, the cutie mark is the self's reflection. The self is supposed to be determined by our choices and experiences. Yet ponies had the same cutie mark in spite of living completely different lives? It's a mockery of free will Twilight... we're just puppets. It HAS TO END!"

Twilight slowly shook her head. "So that's it? You give up on everything you've seen and learned? Just like that? You go back to slapping an equal sign on everypony and think it'll make them happy?"

"No equal marks, no cutie marks, we'll all be blank flanks, and we'll be free to choose our fates, instead of them being decided for us. It's the only way Twilight, that anypony will ever be truly free."

Twilight Sparkle gasped in horror. "You can't! Without her cutie mark! Celestia! The sun!"

"Cutie marks don't define our abilities... the way your friends utterly failed at each other's lives showed that. My equal mark SUPPRESSED any exceptional talents ponies had.

"I was so afraid of differences destroying friendships, that I didn't realize what I was doing. If I had simply left ponies with no cutie marks, they have pursued any path they wanted, not just the path fate had given them superpowers for. This way, ponies CAN be what they want to be, not what fate decided they would be before they were even born. Instead my equal marks crushed individuality, and limited ponies in how well they could do something..."

Starlight laughed. "You know! The idea I had originally, was for every pony who was made Equal: they'd just add their talents, skills, and abilities to the whole! Everypony would be equal, because POWER was distributed equally, powerful unicorns like US would be brought down to 'average', but ponies who suffered from sub-par abilities would be BROUGHT UP to normal instead of being crippled! And the 'average' would only keep rising as more ponies added their strength to the pot. The equal marks would all be connected, evenly distributing power among the community, no superiors or inferiors, and no need to crush ponies to below average.

She laughed again. "...But I couldn't figure out how on my own... and because I have such a huge messiah complex, and I didn't trust other ponies to begin with, I didn't even TRY to ask for help on how to make it work! So I took the easy way out instead! And you saw the end result."

"WHY DIDN'T YOU ASK FOR HELP NOW?" Twilight cried.

"... I've been on both sides now Twilight... maybe I am just crazy, maybe I've been crazy since the day I lost my home, or maybe since Sunburst left me without a word, but I think it's given me a new sense of clarity. Or maybe... I was scared to do this.

"This spell is black magic of course... meaning it requires a sacrifice." Glimmer took out a rune covered knife.

Twilight's eyes widened, she took a trot back, and charged up her horn, preparing to defend herself. "Starlight... how can you still hate me so much?"

"Not you Twilight. I can't ask this of anypony else. My idea. My sacrifice. Thank you for everything Twilight."

A great flash of light from Starlight's horn blinded Twilight's vision. When it cleared, Starlight was on the floor, the knife clearly sheathed inside Starlight Glimmer.

"STARLIGHT!"

The unicorn whispered, dimly looking at the Alicorn. "... I hope... you can forgive someday... Twilight..."

Starlight Glimmer's body turned to ashes, and a shockwave of magic covered Equestria.

And across Equestria... cutie marks vanished. Every last one of them.

What happened next, was no longer up to fate.