Just An Average Trip to Equestria

Demon Shimmer grinned as she watched Sunset fall asleep. All she had to do is pop into Celestia's dreams to speak to Nightmare Moon and Daybreaker, then slip into the dreams of those ponies and people who have their own demons, and the plan would be ready.

She disappeared into the dream realm. In this realm, she could escape from the jar that Sunset had sealed her inside. She was surrounded by what appeared to be bubbles. She saw Celestia in one of them and stepped into a meadow, where the sun shone brightly. She went up to the sun, where Daybreaker was hiding from Celestia.
"Daybreaker, good to see you," she giggled.
Daybreaker's fiery mane and tail had been tied in pastel-coloured ribbons, and she was in the process of repairing her damaged armour.
"Sunburn, help me with the ribbons," the evil alicorn grunted.
"Gladly." Sunburn Shimmer untied the ribbons in Daybreaker's mane and tail. The alicorn smiled.
"In the world that Nightmare Moon and I are creating, demons shall rule. In one half, the sun shall shine for all eternity. In the other half, the moon and stars shall never leave. That way, my sister and I can get what we both want."
"I have a slight complication."
"Oh?"
"Sunset took me out of her mind and sealed me in a jar."
"How humiliating that my student got stuck in a jar. Rarity has been... Quieter, from what I have heard. She remains undetected. You could not keep silent for three seconds. Oh well. At least you can operate in the dream realm unnoticed. You could even slip into Sunset's dreams. As long as Sunset does not see you."
"I shan't risk that. Besides, I need to know when is best."

Nightmare Moon and Nightmare Rarity materialised.
"Well, sister, this is interesting," said Nightmare Moon. "Sunburn has actually been of use."
Sunburn gritted her teeth. "I can operate without detection. Luna, Celestia and Rarity know that you are doing something, but are too preoccupied to sort it out. I was just asking Daybreaker when would be best."
"Hmm," said Rarity, "I would say... Next Wednesday. That is when the Moon and Sun will be in the right positions. Isn't that right, Queen Daybreaker?"
"Yes. That is correct. Now, all of us must rally all the other known demons before then. The unknown ones shall follow when we take over Equestria. Friends, spread word, and meet on Wednesday morning."

Eventually, Sunburn found the dreams of the human Twilight Sparkle. She flew to the full moon. "Midnight!" she hissed. Where was that demon?
"Coming, coming," groaned Midnight.
"What's up with you today?!"
"Well, this demon doesn't exactly like talking to the demon of the person who ruined my plans."
"I am Sunburn, not Sunset. We don't see eye to eye. I would have let you tear apart Earth to get to Equestria. Sunset had other plans."
"Good ol' Sunset."
"Next Wednesday, at dawn. It does happen at the same time on Earth, doesn't it?"
"It does. It will be beautiful."
"Good to hear that you approve."
"Of course I approve! Eternal midnight! What is there to disapprove of?!"
"Well, this is Midnight Sparkle, who does not like working with other demons."
"Ah, good point. Still, there are several long-term gains to helping out, such as seeing and causing a lot of destruction, so I'm in."
"Wednesday, at dawn. Be sure to have Twilight under control by then."
"Coming from the demon who got banished from Sunset's mind. That is a problem for Queen Daybreaker and the Mare in the Moon."
"Which I will solve. Curiosity killed the cat. Surely me sitting in that jar smiling for hours on end will make her curious as to why I am happy. Then I will take control."
"Extreme magic?"
"Summer Sun celebration. You'll be taking over then as well. Twilight wants to stay up. She'll be tired. When the Sun rises, she'll have to rely on magic to stay awake against the extreme sleeping potion that you will make sure she drinks an hour before sunrise."
"Yes. Now run along, it's nearly dawn in Equestria. Dawn only collides in the two worlds at the Summer Sun celebration and Midwinter. I have a few minutes to get ready."
"Bye Midnight."
"Bye Sunburn."

Sunset woke up in one of the guest bedrooms of Twilight's castle. It was a Sunday. Her demon sat there, in the jar on the bedside table, grinning like a maniac. No change there.
"Listen, demon," Sunset told it as it turned around and pretended to sulk, "no amount of sulking, crying, pleading, looking bored or grinning manically like you have a plan, will get you out of that jar. You hear me?" It nodded. "Good. I need breakfast." She left the room, leaving Sunburn Shimmer to go back to grinning.