Night had long since fallen by the time Princess Celestia, Rainbow Dash, Daring Do, and Fluttershy returned to the valley, exhausted and weary.
"Sister," Princess Luna said as the weary ponies joined them. "Worry not for your duties; I saw to the setting of the sun in your stead. Did you succeed in retrieving the prism?"
"We have it," Celestia said with a sigh. "But, well..." She levitated it out of her saddlebag.
Luna gasped. "No..." Tears streamed from her eyes as she beheld the dark, cold prism. "Father's soul..."
"I know," Celestia said. "We're too late."
"What do you mean you're too late?" Twilight Velvet demanded. "You got the prism, right? You can...you can bring my daughter home, right?"
Celestia sighed. "I'm afraid not," she said. "Without the life essence of an alicorn, the machine lacks the necessary beacon to guide Twilight Sparkle through time. Father sacrificed himself to be the power source for this device." She caressed the prism lovingly. "Without his life essence, there is nothing that can reach across time to call her home."
* * * * *
Twilight Sparkle stared at the steadily pulsing prism atop the obelisk.
"Mother...come home," Celestia said.
"Not just yet," Twilight said. "Just a bit longer..."
Celestia sighed. "Mother...it's been six months," she said tiredly. "Whatever Father thinks he accomplished...it failed."
"No," Twilight said, shaking her head. "He succeeded. I'm here, in this time. You and your sister live. The...the tomb worked. Inner Light's sacrifice was not in vain. But..." She sighed, hanging her head. "I don't understand...why I'm still here..." She sniffled. "I should...I should have gone back by now..."
Celestia laid a hoof on her shoulder. "I'm glad you didn't," she said. "I don't wish to lose you, Mother."
Twilight turned and embraced her. "Nor do I wish to leave you, Tia," she said. "But I have to." She smiled sadly. "I have a life to return to. Friends who need me. We have important things to do..." She shook her head. "The ponies of my time need me and my friends. My friends need me. And I need them..."
A long silence passed.
Just as Celestia turned to leave, Twilight gasped. "My friends!" She shot out of the tomb, flying down the tunnel into the valley.
"Mother! Wait!"
* * * * *
Princess Celestia gently floated the prism into its proper place at the top of the obelisk. Nothing changed; it remained dark, and the sun stones around the room remained dim. She sighed. "Rest in peace, Father."
The others watched her silently.
"So...what now?" Rainbow Dash asked. "We got the prism thingie back, but...but we're never gonna see Twilight again?"
"I simply can't accept that," Rarity said. "There must be a way...!"
"C'mon, y'all," Applejack said. "We gotta...we gotta buck up...Twi wouldn't want us like this..."
"I'm sorry, AJ," Pinkie Pie said sadly. "I don't...really feel much like smiling and laughing right now..."
"Princess Celestia? Princess Luna? Can't...can't you do anything...?" Fluttershy asked.
Celestia sighed. "I...I'm sorry, Fluttershy. Everypony. I just don't—"
Luna gasped. "Celestia! There is a way!" She grasped Celestia's shoulders, excitement shining in her eyes. "Don't you remember? The day Mother left?"
Celestia blinked rapidly. "That day..." Her eyes widened. She looked at the other ponies. "Of course..."
* * * * *
Celestia and Luna watched curiously as Twilight Sparkle flew above Ponyhenge. Its six marble towers, each shaped like a giant stone chair, stood in a circle around a broad, flat stone mesa.
"I'm an idiot," Twilight said, facehoofing as she stared down at Ponyhenge. "It was right in front of me the whole time!"
"What was, Mother?" Celestia asked.
"Ponyhenge...it's the Cutie Map. And the thrones from my castle." Twilight shook her head. "And...I completely forgot about the engravings..."
Her horn glowed, and six beams of magic spread out from it, each striking one of the marble towers. Glyphs appeared at the top of each, inscribed in the ancient language of the alicorns. Celestia and Luna watched, puzzled, as each glyph formed:
HONESTY
KINDNESS
LOYALTY
GENEROSITY
LAUGHTER
MAGIC
Twilight's horn fell still. She descended to the mesa, spreading her wings. A bright smile lit her face.
Each of the six towers began to glow. A loud, ringing chime filled the air. A wind picked up on the plateau.
"Mother...?" Celestia asked.
Twilight opened her eyes and looked at her daughters. "Take care of yourselves," she said. "We'll see each other again..."
* * * * *
Applejack, Fluttershy, Rainbow Dash, Rarity, and Pinkie Pie each leaned against one face of the obelisk, eyes closed in concentration.
"Call to Twilight Sparkle," Princess Celestia said. "Pray for her return." She turned to Twilight Velvet. "Meditate on your desire to see her again. Think of nothing else except your love for her."
A wind stirred the mares' manes and tails. Their Cutie Marks began to shine. The prism began to strobe in six different colors.
From the corner, Spike, Princess Luna, and Daring Do watched in awe.
All around the tomb, the sun stones flared back to life. Golden runes undulated from the very walls of the tomb, flashing and pulsing as they writhed and swam in the air.
* * * * *
Six beams shot from the spires of Ponyhenge, each a different color. They converged on Twilight, coalescing into a glowing rainbow tornado before shooting into the sky, a bright white pillar of light.
When the light faded, Twilight Sparkle was gone.
Luna began to weep. Celestia wrapped a wing around her.
"Safe journey, Mother...we'll miss you."
* * * * *
A column of white light burst through the ceiling, slamming into the prism, which bent it toward the far wall of the tomb. A brilliant flash, like lightning amplified tenfold, filled the room.
When the light faded, the group of ponies stared at the tall, regal figure that stood there.
"Twilight...?" Twilight Velvet asked softly.
"I'm back," Twilight Sparkle said.
"TWILIGHT!" Five mares charged her, engulfing her in a giant group hug. She laughed and hugged them all back.
Velvet approached curiously, one hoof raised. "Twilight? Is...is that really you?"
Twilight smiled. "Hi Mom," she said. "I...I guess I've...changed a bit, huh?"
"Hay yeah!" Rainbow Dash cried. "Geez, Twilight! What's up with you and getting blasted to atoms by crazy magic, then we all get worried about what happened to you, and then when you show up again you've gotten all princessy on us?" Tears streamed from her eyes as she laughed.
"It...does seem to be a recurring theme with you, darling," Rarity said with a tearful smile. She studied Twilight critically. "Hmm. I suppose I'll have to fashion you a completely new wardrobe...again..."
Princess Celestia and Princess Luna slowly approached, wonder on their faces. "Mother," Celestia said softly.
Twilight looked at the two ancient alicorns, tears in her eyes. She smiled. "Celestia...Luna..."
She sniffled.
"You're grounded."