Twilight fired a bolt of magic at the mare claiming to be the princess of the sun. Celestia expertly dodged the energy bolt, "Twilight! I am not here to cause you harm! I am here to ask for your help in finally destroying my sister!"
Twilight stood in a defensive pose, horn primed and ready to fire, "My friends and I have already cleansed Luna with the elements! Nightmare Moon is gone! The real Princess Celestia would know that, imposter!" She fired another bolt of energy.
'Celestia' dodged the bolt again, "Not Luna. I speak of my other sister, Mystica. She is the alicorn of magic, and one of the most evil beings I have ever faced." The regal mare lowered her head, "I too have fallen victim to her deception. I am a prisoner here just as you are."
Twilight was still cautious, "Just where is 'here' anyway?"
Celestia replied simply, "This is the Spiritual Plain. The land of souls who refuse to move onward. We are able to remain here because of our connection to the Material Plain, where our bodies remain but belong to us no more. Your raw magical power has prevented you from disintegrating into nothingness, it also keeps you tethered to your body. This is why you can see what Mystica sees."
Twilight took the information in, "Okay. Then why are you also trapped here? And why is your mane pink instead of it's normal rainbow?"
Celestia sighed, "That is a very long story, Twilight."
Twilight sat down, "I have time. We're not exactly going anywhere."
Celestia sat next to Twilight, "Fine. It all began over two thousand years ago with the banishment of Mystica. Originally, each of us wielded two of the Elements of Harmony. I wielded kindness and laughter, Luna wielded generosity and loyalty, and Mystica wielded magic and honesty."
The ancient castle of the royal pony sisters, the palace that was home to Equestria's royalty. Mystica stood in the throne room, "I say we crack down on these subjects! They need to fear us! We wield the most powerful artifacts ever known! If we assemble an army we can conquer the rest of this land and rule the entire planet! There is nothing to stop us!"
Celestia stepped forward, "Nonsense, Mystica! If our little ponies fear us, they will never respect us. And our kingdom has plenty of land, there is no need for us to start wars with the neighboring countries that are just starting to grow."
Mystica stomped her hoof, "Exactly why we should strike now! They are weak and will fall easily! Maybe I should relieve you af your crown, sister. You might not understand what it takes to rule. Right, Luna?"
Luna sat in the corner of the room, being the youngest, she never really go have opinion. She just got to agree with whichever sister scared her more. Which was commonly Mystica. Luna meekly replied, "I-I guess. I don't want people to be afraid of me though, I want other ponies to like me."
Mystica turned away from her sister, "Your mind is to young to understand. Maybe you are not worthy of your crown either. When you have power, you need to use it. They should see us as terrifying gods! Not as normal ponies they can befriend!"
Celestia glared at her sister, "And what do we gain from that? Tell me, Mystica. If our ponies fear us, then they could revolt! They would not listen to us! They would not care about us!"
Mystica began to get angry, "And if they revolt, then we will smite them where they stand! Maybe I should rule Equestria alone since I'm the only one with any sense here!" Her horn began to light, "I shall rule Equestria as queen! I shall lead us to a new age of prosperity! And you, dear Celestia, shall bow before my hooves!"
She fired a beam of light towards Celestia and Celestia countered with her own beam of energy. She began to strain under the power, "Luna! Get somewhere safe! Warn the ponies of Equestria that Mystica has gone mad!"
Luna nodded and quickly flew out the window, alerting the city to the dangers about to come.
Back in the Spiritual Plain, Celestia sighed and stood up, "Mystica escaped. She brought army after army to challenge Luna and I for what she claimed was her rightful crown. Occasionally a pony would escape from her ranks and flee to us. They would speak of tortuous conditions and the terror brought on by my sister. They would ask for forgiveness and I would give it, of course. If it was not for the elements deeming Mystica unworthy, I'm afraid we would have never defeated her."
Celestia began to walk around, "I thought we had defeated her for good, but when I banished Luna, she struck. Apparently she still had a small connection to the Element of Magic. When I used the elements on NIghtmare Moon, she took that connection and forced herself into my mind. My conscious was now hers, the surge of magic changing my mane from pink to the pastel rainbow you see it as now and forcing mot of my mind to the Spiritual Plain. Since then, Mystica has used me and my powers to find a suitable host for her soul, eventually finding one in you, Twilight. She saw the raw magical power in you and began to groom you for the possession."
Celestia smiled slightly, "I watched from here. You were everything that she was not. You were kind. I knew that when the time came, I would save your soul. Because I believe that you can help finally eradicate the poison that is Mystica. You have the power to best her, Twilight Sparkle. We just need to get you ready."