Dear Princess Celestia,
This will be a long letter, but all of this needs to be said. Soren has been gone for three days, and no pony can seem to find him. Your sister has been gone for quite some time as well. Where is she? We could have used her help yesterday; there was a forest fire at the edge of Ponyville. I was in my library, reading a fantastic book about the history of Cloudsdale, when Rainbow Dash kicked the front door open and urgently said that she needed my help.
The forest fire was at the edge of the Everfree Forest and slowly grew in the direction of Ponyville. On the way, Rainbow Dash explained that the fire was too strong for the weather pegasi to stop alone. As we neared the edge of town, I could already see many ponies running back and forth, collecting water to put out the fire. The fire was massive when I saw it. Luckily, I had already thought of an idea on how to stop the fire as we ran through town.
I looked at Rainbow and asked her to gather as many pegasi as possible so they could create a tornado to absorb the fire. As she flew off to do that, Applejack came running up to me. She told me that she saw a hooded pony go into the forest. I explained to her that while the pegasi and I worked on the fire, her and Rainbow should create a very small search party to find this hooded pony and bring it back to town for questioning. She nodded and ran off to find Rainbow Dash.
Eventually we managed to put out the fire by bringing the fire tornado to the local reservoir. Afterwards, Fluttershy and Rarity came up to me to inform me that Applejack, Rainbow Dash, and Pinkie Pie were chasing the hooded pony around town. We eventually managed to chase down the hooded pony. (Technically, Pinkie shot her party cannon at the pony, hitting it off of a roof.)
When we surrounded it, the pony began to speak. It was stallion, but what was strange was that he seemed to recognized us. (Also Rainbow Dash pointed out that this pony was a unicorn.) The strangest part, though, was how he talked; he only opened one eye at at time as he talked to us. (We couldn't see his face, just his glowing eyes.) One eye was glowing yellow and the other was glowing grey. As he switched eyes, his voice changed, almost like he had two ponies in him. One voice sounded slightly sinister, the other voice...sounded a little like the Doctor. I knew it wasn't, but that still raises the question: Who was this pony?
I saw that Rainbow Dash was ready to pull that hood off of the stallion, but I stopped her the first time. We had to know more about this pony, and if we became hostile, we could scare him away and never find him again. The strange became stranger as this pony called me Ms. Sparkle (just like the Doctor does) and began gloating about how he started the fire. He seemed to know that we were about to capture him because he chuckled and said the one thing I will forget: "The only thing that stands in your way from stopping me is a close friend of yours."
Although we were baffled by this, Rainbow Dash took the opportunity to fly straight at him. The stallion stepped to the left at just the right moment, then he hit Rainbow's side, which sent her a few feet away. After that he said that it was time for him to leave, so he started walking away, but we were all frozen there, amazed by what this pony easily did to Rainbow. He didn't run away...he teleported. Princess, we are trying our best to find this mysterious pony. Wish us luck.
Your faithful student,
Twilight Sparkle
---------------------
Three Days Earlier
---------------------
The Royal Palace is usually very quiet during the night. The only noises that are heard are the hoofsteps of the nighttime guards during their patrol. Other than that, there is almost no noise to be heard. However, on this night, a strange sound could be heard coming from Princess Luna's bedroom. It only lasted a few seconds, but the noise was unmistakable to the few ponies that have heard it before.
A few papers flew around the princess' room as the TARDIS landed inside. Once the TARDIS completed its landing, the Doctor, with Princess Luna on his back, stepped outside. He carefully made his way to the princess' bed so he could place her on there. Once Luna was laying on her bed, the Doctor frowned.
"Why did you do it, Luna?" he asked no one. He knew the alternative wasn't better, but she's a goddess for her sister's sake! The consequences could be much more dire. A knocking came from the bedroom door, breaking the Doctor's train of thought.
"Luna, are you in there? It's your sister." The Doctor walked over to the door to let Princess Celestia in. She looked the Doctor, looked at Luna, and became worried. "Doctor, what happened to her?"
"I think it would be best if we let her wake up first. Come in." The goddess and the Time Lord stood on both sides of Luna's bed as Celestia attempted to wake her sister up.
"Sister, can you hear me?" Celestia said softly as she gently nudged Luna. "It's time to wake up." The princess of the night groaned and slowly opened her eyes. Upon seeing her sister and the Doctor, Luna smiled.
"Hello, dear sister, dear Doctor. What do I owe the pleasure of this visit?" The Doctor looked at Princess Celestia.
"Let me handle this. Princess Luna, do you know what happened earlier today?"
"Is it not day?" asked a confused Luna. She sat up in her bed and began pondering the Doctor's question. "No, I cannot say I remember the events from earlier. Have I missed something important?"
"There was a battle between Soren and the Nightmare Guardian today," Celestia said while guessing what became of her sister. "He helped to save Canterlot from her."
"The Nightmare Guardian was here?! Who is this Soren stallion who apparently has the power capable of defeating her?" Celestia hung her head. Her guess was correct. Luna had truly lost her memory. Not just of earlier today, but of Soren.
"I will explain everything tomorrow," Celestia said. "For now, you should go back to bed, sister. The both of us will be doing the same."
"Actually, my TARDIS is still in her room," the Doctor pointed out.
"You can spend the night in the palace, Doctor. Let us leave Luna to rest." The Doctor reluctantly agreed and followed the sun princess out into the hall. He walked by her side as the two walked further away from Princess Luna's bedroom.
"How many days has she forgotten, Doctor?" Celestia asked.
"From the day she met Soren until now," he answered seriously. "The spell she was supposed to use on Soren backfired and caused her to lose her own memories of him, including everything that happened on the days she was with him. She chose to do this to herself. Instead of erasing Soren's memories of the past few months, she erased her own."
"Is Soren's magic still unstable?"
"Not yet. Since Luna refused to follow through with the spell, she simply halted his magic from growing out of control. I'll give him one or two weeks before his magic overwhelms him."
"Does Soren know about this?"
"...No. There's nothing we can do for him at this point. Sadly, he's on his own."
"...I believe you. You would not sound so serious and sorrowful if you had a plan. I only wish there was something we could do."
"There isn't. We will just have to see what happens in the next few days." The princess and the Doctor remained silent as they continued through the dim, quiet halls of the Royal Palace.
------------------
The Next Day
------------------
The morning after the Doctor left, Cain began speaking to Soren again. At first, Soren simply tried to ignore the voice in his head, but he eventually found himself unable to. Cain began explaining to Soren how he was the only one that could help Soren in his current predicament. Cain explained that Luna's spell didn't heal Soren; Soren still had chaotic, unstable magic waiting to be unleashed. Cain predicted ten days at most. Soren asked Cain if there was anything he could do since Cain was so smug that he could help. The spirit said there was, but it required them to make a deal.
If you help me with one, only one, big plan of mine, I will heal you and leave you alone.
'What's the catch?'
We join forces, and by that I mean I get to come back into your body.
Without Soren's consent, Cain started to change Soren body slightly, due to another "pony" entering. This time Soren could feel his eyes changing and he could also feel Cain trying to take control. Soren fought back hard enough for Cain to stop his spiritual assault.
"Oh? You're stronger than the last time we met. Very well. In all seriousness though, I will not attempt that again since it looks as though we have even control of your body."
"Wait...you can use my mouth to physically talk now? Something else feels off as well."
"It would appear that we can both use your mouth, yet I can only see out of one eye, which is the left one."
"Weird, I can only see out of the right eye. Maybe only one eye can be open at a time? I guess we really do only have fifty percent control."
"Back to business. Do you want to practice your fire magic?"
Soren raised his right eyebrow. He still doesn't trust Cain, but it looks like he really doesn't have a choice at the moment.