I started writing Mortal long before season 3 started and rumors of Twilight's ascension began. The opening was somewhat different. The original prologue is below, followed by the first scene from chapter 1.

The carriage wheels tore up small clumps of sod as they rolled through the cemetery, slowing to a halt before a row of pristine marble tombstones. The unicorn pulling the carriage shrugged the harness over his rich vermilion coat and, with a heavy sigh, let it fall to the ground. His breath steamed faintly in the cool air.

The unicorn ambled to the side of the carriage and opened the door. “We’re here, Mom,” he said.

“Thank you, darling,” came the response from inside. “Help me down, please.”

An elderly white unicorn tottered out of the carriage, leaning heavily on her son as she went. Her withered legs were barely thicker than her horn, except for the swollen joints at knee and shoulder. Her mane was the same arresting violet it had been in her youth, but the chemicals Rarity used to maintain its color and shape had left the hairs thin and brittle. She wore a heavy coat and matching shawl against the cold.

On her other side, she was supported by an old friend. Fluttershy’s body showed few signs of her age; although her mane had faded to a rose-tinted white, she moved with all the grace and power of a mare of forty.

Behind them, a rolled-up blanket and a wicker basket floated in a nimbus of azure light. Rarity’s magic, at least, was as strong as ever. She leaned on Fluttershy while she telekinetically spread the blanket in front of a tombstone. She made her slow way over, supported by the ponies on both sides, before they gently settled her onto the blanket. Fluttershy opened the basket and produced a chocolate cake, a bottle of champagne, and a pair of bright orange party hats.

The younger unicorn stood up. “I’ll be at the fountain when you need me.” He turned to go.

“Sunstone.” Fluttershy put a hoof on his shoulder. “It’s so sweet of you to take us out here.”

“I know how much this means to Mom. To both of you. I’m happy to do this.”

“One of my grandfoals should be helping you, at least.”

“They’re busy. It’s apple bucking season, and they’ve got to worry about the wedding in a couple of weeks, too.” Sunstone wiped the sweat from his brow. “Don’t worry about me. I’m not too old to pull a carriage just yet.” He trotted off, leaving the two old friends alone.

Fluttershy slipped the garish hat on Rarity’s head before putting on her own. “This is the smallest birthday party we’ve had,” she said.

Rarity’s teeth were clenched tight. She magically popped open the bottle and poured two flutes of champagne. Two more glasses sat untouched in the basket. “You know Rainbow Dash doesn’t like to come here anymore. Not since Applejack died.”

Fluttershy nodded. “Still, for Pinkie’s birthday… well, I hoped.”

“It’s her decision.” Rarity’s voice was firm, but she looked at the ground as she spoke. “I do wonder where Twilight is, though. It isn’t like her to be late.”

“Oh, that’s right. She left this.” Fluttershy reached into the basket and produced a scroll. Rarity floated it across the blanket and unrolled it.

My dearest friends,

I’m so sorry, but I’ll have to miss Pinkie Pie’s birthday party this year, as Princess Celestia has called me to Canterlot. I explained how important this day is, but she insisted. I’m sure she has a good reason. Make sure to save me a piece of cake.

In Memoriam,

Twilight Sparkle

Rarity frowned. “The princess had better have a marvelous excuse for this.”

“Let’s not get angry,” said Fluttershy. “Not today.”

“I suppose not.” Rarity straightened her shoulders. “Shall we begin?”

Fluttershy nodded. She set several candles in the cake and lit them. Rarity looked up at the tombstone. She swallowed once, then again, and began to sing. Softly, Fluttershy joined in.

“Happy birthday to you
Happy birthday to you
Happy birthday dear Pinkie
Happy—”

Both ponies pulled back as two bursts of magenta flame sprang into existence before them. The wind whipped the hoof-sized firebursts away, leaving two scrolls to fall to the blanket, one in front of each pony. The messages were sealed with purple wax bearing the mark of a six-pointed star.

The friends exchanged a look, then opened the letters. A quick glance confirmed that the contents were identical.

I have big news. Meet me at the library as soon as you can.
-T.S.

Rainbow Dash flew low to the ground, Twilight’s mysterious message still running through her head. She longed to put on a final burst of speed to reach the library door, but fought down the urge. The doctors all said that if she pushed herself too hard, her wing might give out entirely and she’d never fly again. That would be—

She stopped herself. She couldn’t even think about it. And so she flew slowly, as though the excitement wasn’t burning a hole in her chest. They hadn’t gone on an adventure since… Sun and Moon, it must’ve been fifteen years, at least. It was before Pinkie’s grandkids found their cutie marks, anyway.

She banished thoughts of the past as she landed and pushed her way into the library. Rainbow Dash barely had time to notice Rarity and Fluttershy, propped up on cushions in the middle of the room, before she found herself shouting for joy and leaping forward, ignoring the twinge in her shoulder.

Rainbow Dash buried her face in Twilight’s neck. “Stars above,” Dash gasped. “It’s incredible.”

Twilight returned her friend’s nuzzle. “It is rather a lot to get used to,” she said.

“Let me look at you.” Rainbow Dash pulled herself away and took a step backwards. Twilight Sparkle had changed. Her mane and coat glistened faintly with an inner light. Her horn was longer and finer, yet it somehow seemed more solid, more real, than ever before. And those wings. Sun and Moon, those wings. They were graceful, powerful, sleek… even during her prime, Rainbow Dash would have done anything for wings like those.

Rainbow Dash looked her friend up and down for well over a minute, trying and failing to keep from giggling at the sheer joy of seeing Twilight Sparkle looking so incredible. Finally, she spoke. “You got young.”

It was true. Twilight’s body was strong and lean. She stood straight and tall for the first time in years. The wrinkles were gone from her face, her coat had regained its luster, and the vibrant streak of rose had returned to her mane. More than that, she carried herself with the energy and confidence they had all lacked for decades.

Twilight grinned sheepishly. “Technically it’s ageless, not young.”

“But, what is…” Rainbow Dash paused, searching for words and finding none. “I mean, what?”

“I was just telling the others,” said Twilight. “Princess Celestia told me she’d always hoped I’d be ready for this, and that now she’s confident I have what it takes, and then she helped me look within myself and find the potential to change, and then… and then this.”

Rainbow Dash felt like her grin was going to split her face in two. “Oh my gosh. This is… oh my gosh. I’m so proud of you.”

“We all are,” said Fluttershy. “You’ve worked so hard for this.”

“So how does it work, exactly?” said Rarity. “Any heavenly powers?”

“I’m not actually sure yet,” said Twilight. “There’s a lot more that Princess Celestia needs to tell me, but I just had to teleport back and show you girls before anything else. I’ll tell you the details as soon as I can.”

“Man, who cares about that?” said Rainbow Dash. “What’s important is that you’re a princess from now on. I mean, you’ll be a princess forever!”

“Oh, my,” said Fluttershy. “I didn’t even think of that. Do we have to call you ‘Princess’ now?”

“Well, you don’t have to,” said Twilight. “But yeah, it’s ‘Princess Twilight’ to most ponies, I guess.”

Rarity wiped away a tear. “Your brother would be so happy if he could see this.”

“Heh.” Twilight smiled wistfully. “He’d be the only member of the family who wasn’t an alicorn.” She sighed. “I still miss him. I guess I’ll have to get used to outliving ponies.”

“That reminds me,” said Fluttershy. “We brought back the cake, like you asked.”

“What cake?” Twilight shot her a blank look. “Oh, wait. Right. Thank you, I’d love some.”

Rarity levitated the chocolate cake out of the picnic basket resting by her hooves. “What about you, Rainbow Dash? Care to join us?”

“I dunno.” Rainbow Dash shifted her weight from hoof to hoof. “You guys know this is a little creepy.”

“You didn’t have a problem with it last year,” said Twilight.

Rainbow Dash’s head whipped around to face Twilight. “Things were different last year.”

“Quite right,” said Rarity. “We’ve always had this little memorial with five of us, before. Going down to four is bad enough. Please don’t make it three.”

“It’s what Pinkie would want,” said Fluttershy. “She’d hate for you to be sad today.”

“Okay, fine,” said Rainbow Dash. “It’s still weird, though.”

This next scene occurred early in chapter 1, after Twilight's first conversation with Celestia.

“There you are.” Twilight opened her eyes to see Princess Cadence. “I see Celestia’s started your training already.”

“Yeah,” said Twilight. “It’s tough, though. I don’t think I’m making much progress.”

Cadence nodded. “I remember how hard it was when I first learned. Here, come with me. You look like you need a break, and there are other parts of being an alicorn that you need to learn about.”

Twilight followed as Cadence made her way to the balcony. “Like what?” she asked.

“Like this.” Cadence unfurled her wings and leapt over the railing. She flew in a short curve to face Twilight again, hovering in place. “Come on out here, and I’ll teach you some of the basics.”

Twilight peered over the balcony’s edge. The Gloaming Tower extended past the edge of the cliff that supported Canterlot. She found herself looking down a drop of several thousand feet to the gently rolling hills below. “Maybe I should stay over the balcony.” Twilight flexed her wings. “I haven’t actually used these things yet.”

“Not once?”

Twilight shook her head. “I’ve been more focused on the magic side of things. My unicorn magic is so much more powerful, and there’s alicorn magic to explore, too. I just haven’t had time.”

“We’ll have to fix that.” Cadence alighted atop the railing. “The first thing is to make sure you beat your wings in time with each other. If they’re not coordinated, you won’t stay level. Go ahead, give it a try.”

Twilight tried. It took several attempts, and Cadence had to correct her form more than once. Her wings still felt foreign, like they were a part of somepony else’s body that had been accidentally loaned to her. Eventually, however, Twilight managed to heave herself off the ground.

“Hah!” she cried. “I’m flying! I’m actually flying!” Twilight hovered perhaps a foot above the balcony—or attempted to hover. She tilted and jerked from side to side in her fight to stay upright. It didn’t feel weightless, like she’d expected; instead, it felt as though she was constantly clawing her way upwards while gravity hauled her down. A heady, invigorating feeling overcame her, like galloping across an open plain and scaling a cliff face at the same time.

“Good job,” said Cadence. “You always were a quick learner.” She hopped off the railing and hovered in the open air. “Now follow me. I’ve got plenty more to show you.”

“Are you sure? I mean, I’m still new at this. Falling to my death is not part of the plan.”

“You’ll be fine. You’re immortal, remember? I’ve put myself back together after worse scrapes.”

Twilight grinned and flew her shaky way beyond the balcony. Cadence folded her wings and dove. With a whoop of awe and terror, Twilight followed.