It had taken some works, but Luna and Celestia were able to emerge the princess of the sun into the dream realm just like her sister. They had found a method that unicorns ,who worked under Star-Swirl the bearded, used back before Luna took control of Equestria's dreams. It was a form of mediation that allowed Princess Luna to draw out Princess Celestia's mind and submerge her into the dream realm. Needless to say, Celestia was in awe at the world that her sister visited so often. She viewed all the different bubbles that held every pony's dreams, even recognizing a few of them herself.
The princess of the sun followed close behind her her sister, but could not stop her wandering eyes from darting around to familiar ponies. She saw Twiligt dreaming about opening up a school for teaching unicorns, which made Celeatia beam with pride that her pupil would want to continue her work. She then viewed Applejack dreaming of being in another rodeo, winning all kinds of 1st place ribbons. Fluttershy playing with giant bunnies while ridding a giant butterfly. Rainbow Dash, Celestia could have easily guess, was dreaming of flying with the Wonderbolts. Rarity was being admired by a large group of ponies for some fascinating new dress she was wearing. And Pinkie Pie was swimming in a bowl of chocolate pudding while cupcakes rained down from the sky. It brought a smile to Celestia's face, she had almost forget her purpose for coming into her sister's domaine. That was, until she saw it.
Her sister stopped about 3 feet away from a bubble the same size as the other dream bubbles, but unlike the others that allowed outsiders to view its entire contents, this bubble was dark shade of stone, and the feel coming from it just as cold. Celestia saw what her sister meant by strange and troubling. As she stretched her hoof out to it, the solar princess was shocked by some electricity the dream was emitting.
"Do you see what we mean, dearest sister?" Luna asked, taking her spot by her elder sister's side. "We have never seen anything like this before, not in all the years we have spent protecting the realm of dreams."
"And you have no idea 'who' could be having this dream?" Celestia asked, her eyes looking over the orb, taking in every detail as Luna had done when she first came across the oddity.
Before Luna could answer, though, a crack began to grow from one side of the dream, and threw it a two voices could be heard. Celestia and Luna where still and silent as they tried to pear in through the crank and listened intently.
"Yeah... I want to protect everypony..." A stallion's voice spoke out, the voice was muffled.
"Ah know.... Ya can.... Yer a good stallion.... Good stallion.... Good stallion..." A filly's voice echoed after the stallion. It was one the two sisters swore they had heard before, but it too was muffled, making it hard to recognize.
Taking a deep breath, Celestia began to approach the crack, hoping to get a better look. From where she and Luna stood, all they could see where shifting shadow. But as she approached the crack, a chilling voice began to chuckle, one that belonged to a stallion Celestia thought was long since gone.
"My my, how are you, dearest Celestia?" The deep stallion voice echoed, sending a chill down both of the princesse's spines.
The sun princess moved back quickly, flapping her mighty wings to retreat to a safe distance. "No.... No it can't be!"
Moving through dream, looking through the crack was a single green eye, with a red iris, and at the edge of the eye burned a small purple flame. The voice chuckled as the single eye darted between the two alicorn sisters.
"Sombra?!" Luna growled. "But how?! You were destroyed at the Crystal Empire when Cadence used the power of the Crystal Heart!"
Sombra chuckled "Put out of commission, broken into a million pieces, but not gone. What's the matter?..." The eye focused on Celestia. "You aren't happy to see me? Aren't you glad I'm alive?"
"Where are you hiding? Why don't you reveal yourself!" Celestia yelled, her heart racing, never expecting to come face to face with the tyrant unicorn ever again. At least, she had hoped she never would.
"All in due time. But for now, a parting gift;
The Sun did rise,
The Sun did fall,
The Sun will rise again,
Bring with it the shadows long concealed,
The king will rise at the prince's fall,
True intentions be revealed,
Ponies gather at twilight's hour,
Not Sun nor Moon not Heart of Gem can save,
The pony of power,
From the path that destiny had cast,
His ship now sails,
Never slowing, ever fast,
Amongst a land or red and green,
Sweet and warm as honey and tea,
Under your watchful eye, dearest Celestia,
The pony in question is never seen.
The two sisters listened to the shadow king's words, repeating them over and over in their heads, but nothing about it made since.
"You truly have gone mad, haven't you?" Celestia finally said, looking down on the single eye of the former ruler of the crystal empire.
Sombra just chuckles in response. "If that is what you wish to believe. It is too soon for you to do anything anyways. I wonder... Will you be too late this time as well?"
As princess Celestia was about to ask him more about what he meant, the stone like bubble he was trapped in became transparent, it continued to do so until the stone bubble disappeared altogether. Celestia was slightly stunned. As she looked around, she saw that the other bubbles were disappearing as well.
"They are waking up" Luna explained, knowing how this realm worked, more so then her dumbstruck sister. "Sombra must have woken up as well."
Celestia thought for a moment, then sighed. "We need to put the guards in the look out. Both in Canterlot and the Crystal Empire. Keep an eye out for Sombra. If he makes a move, I want to know."
"We agree, sister" Luna said as she turned around and watched the other dream bubbles begin to disappear. "We should head back, it's time for us to lower the moon and raise the sun."
Celestia paused. 'Pony's will gather at twilight's hour...' She repeated his mad words over again. "Luna... What if he wasn't just speaking nonsense?"
"You think his poorly written poem might hold some truth to it, sister?" Luna asked, turning back and facing her older sister, concern spread heavily across her muzzle.
"I believe it might be worth looking into. When we get back I want to send a message to Twilight Sparkle, if anypony other then us can decider it, she can" Celestia walked up to her younger sister. "I'm sorry... But I'll have to ask you for your help on this once more..."
Luna smiled and put a hoof on her sister's shoulder. "We are always here for you sister. We know.... Facing Sombra won't be easy for you... But you will always have us by your side, dearest sister. "
The lunar princess proceeded to give her older sister a hug, which Celestia gladly accepted. A sharp and ancient pain burning in the solar princess's heart felt like it was reopening. The two stayed like that until Luna broke the meditation spell that held them both in the dream realm. In a flash, both Luna and Celestia returned from the realm of dreams. But the pain still lingered in Celestia's heart. One that she had done her best to forget about for 1000 years, with little to no success.