Princess Dinky's Letters: A Magical Secret

Dear Fillies, I have just learned a dark secret about somepony who I thought was my friend, and I realise now that I cannot hide the truth from anypony. This letter is about some other magical secrets I discovered after earning my cutie mark.

After school on that day I earned my cutie mark, I raced home to find my Mummy. She was baking a batch of oatmeal muffins.
"Mummy, Mummy, look at me!" I cried. She turned, focused her retinas for just a moment on my flank, and her eyes grew wide as her retinas slipped to the sides again. She shook her head and mentioned to The Door That We Never Used. It was the door to the cupboard under the stairs. I opened it and, to my surprise, another staircase was revealed. I looked at Mummy. She nodded. I walked down. It was darker than midnight when the moon is new. I heard Mummy walk into the cupboard and snap the door shut. I found the bottom of the staircase. I stepped to one side so that Mummy wouldn't walk into me. She came to the bottom and said, "Muffin, muffin, muffin." The only word she could say was "Muffin," so this didn't worry me. What did was the little bubble of blue light that floated towards her. She opened her mouth and swallowed it and her throat began to glow blue. She coughed and did something she had never done before: she hummed.

It took a few moments of listening to her humming before I put two and two together and said to her, "You can speak now."
"Clever girl," she replied. I tried no to scream with delight and surprise. "But you are not as clever as you need to be."
This confused me. "Why?"
"Flank."
"Why? What's wrong with my flank?"
"It's got a cutie mark."
"And?"
"Fire. Shadows. A bubble to represent family. It shows power. Cosy Glow could get some idea that you're going to start a rebellion."
"Oh."
"Use your magic and think conceal. It will let you hide your cutie mark." I thought conceal and it worked. My flank was blank! Then I thought uncover and my cutie mark appeared. Then I thought conceal again and it was gone. Mummy burped and the blue light came out of her mouth and disappeared. She was mute again. We walked back up and found ourselves in the cupboard and opened the door and Princess Cosy Glow was there.
"Hello Derpy, hello Dinky, what are you doing in a dark mouldy cupboard under the stairs?" she questioned innocently.
"There was a Changeling in there. We had to get it out and it turned into a filly called Diamond Tiara - I think you knew her - and it ran off."
I could hear her thinking, She's lying, they're hiding something, if only I could get that Derpy idiot to talk. Torture could work...
"Please don't do anything to us!" I blurted out. I realised I had made a great mistake. She looked curious for a moment. Only she and I knew, but I had just read her mind.
"How did you know I was taking you to prison and questioning you?"
A lie was so easy. "You do that too often."
"Hmm, interesting." I could hear her thinking again: Twilight Sparkle is behind this. I could really do with seeing her turned into stone. Or dust. The problem is, how? Twilight's a blank flank. So Twilight had invented the spell to hide and reveal a cutie mark.

Cosy teleported all of us into a prison cell. The three walls were made of stone. There was no door or bars to cover the gap that showed the corridor. It was a gap. I darted through and found myself running through invisible slime that turned solid. I was stuck. I could still use magic, but my flank was no longer blank. I read Cosy Glow's mind. Hmm. Fire and shadows. Could incite a rebellion. But she's a filly. Plus, her butt is stuck in the invisible wall.
"Dinky," Mummy called. I looked at her. The blue light was in her throat. "It's time you knew the truth. The Hiding Spell can hide a lot more than just a mark. Including being Princess Luna's daughter." My eyes grew wide. I was Princess Luna's granddaughter. I noticed that she was surrounded by blue light and was growing taller. Her blue wings became huge, her neck and legs grew long, and a horn spiralled out of her head. The light disappeared and there she was, Derpy Hooves the alicorn.

"I can't use my magic," she said, "but half of you can. More specifically, your top end." I looked at myself. I had sprouted a pair of wings that drooped uselessly. I made my horn light up and heat the invisible wall. It turned into slime again and I dragged myself out. Mummy was not so lucky. Cosy Glow teleported her somewhere and I didn't see her again for five years. I tried not to cry, hid my wings and cutie mark and ran to the secret base for the School of Magic and tried to find Twilight Sparkle, but she was gone. And by gone, I refer to the secret that I found out today.

Twilight has a Nightmare Pony, an evil version of herself who takes over when she is consumed by her negative emotions. During the rebellion, she let this Nightmare Pony take over. As far as I know, it has no name, but you will have heard of her. She is the Shadow Pony. As I write this I can imagine you gasping. I was just as shocked. I would like you to write back because I don't know whether to share this or not. Currently I am just disgusted that she did not tell me before. Still, she is my fellow princess. I cannot let you share this secret yet.

I hope you are all well.
Yours sincerely,
Princess Dinky.