Sweet Blast 
"[@Vortaxonus":](/pony/your-own-episode-ideas/post/3573268#post_3573268
)
I think the beginning of 1st part will be story about the 3rd Princess Celestia sister- Princess Of Weather what having the power of sun and moon,plus she having her own power called weather when one of sisters being sick. She helped too the other ponies with her own power. But one day Princess Celestia gets mad at her older than her sister.
,,You doesn't supposed to be my sister! Y-you ruined anything! You can't work even with all those weather!"
,,Well. See you about it. Celestia."
And then she changes into Weatherbreaker. (Nightmare Dash). Princess Celestia doesn't know what she needs to do. And then. She got an idea. Banished her too. To. Ponyville. Princess Of Weather planned the revenge for Celestia. But. How? Easy. She used her horn and she did a spell what can change her in pegasi or unicorn or even earth pony! She chooses to being pegasi and she changes into Rainbow Dash.
After story Twilight gasps in shock and she asks Rainbow does she's older than Celestia. Rainbow started thinking about lie and she said there's so many ponies what that pony can change and not only she can be. But Rainbow after going home started thinking does that story is real. In next day she founds in her shelf an strange amulet what only in Day Of Sun she can change into Weatherbreaker. (Action in 1st part is day before Day Of Sun and the moment with Rainbow Dash transformation.)
Later she goes to Twilight to asks what is this. But Twilight started again asking her about Princess Of Weather and does she is her. Rainbow can't handle this anymore and she said:
,,Guh! Okay! You got me! I am that Princess Of Weather! Happy now?!"
Twilight gasped in shock and she asks does she dangerous. Princess Of Weather in Rainbow Dash body said:
,,No. Well. Maybe a little bit."
Later Twilight did the same happy face what she did in s7e25 or 26 and she did the spell who can she see in Rainbow Dash body. But the visuals was more crazy than she expected. Princess Of Weather was a white alicorn princess with wavy rainbow mane. Her cutie mark was sun with rain. Anywho. Visuals wasn't longer than few minutes because Rainbow Dash runned away crying and saying:
,,Y-you will never understand me! Even if I am the real Princess Of Weather! S-so. Just leave me alone!"
Later in Everfree Forests after crying short scene she feels a strange feeling in her chest. Weatherbreaker mumbled to herself:
,,Hm. It's getting dark. Better gets outta here until the festival in Ponyville town hall started. And I. Having the special show for everypony."
And she flies away. Later Rainbow heard Rarity voice saying:
,,And now! Let's clap your hooves for Princess Celestia!"
Rainbow screamed:
,,No! You can't do this!"
Other ponies looked at her. Twilight asked:
,,Princess Of... I mean. Rainbow Dash? Are you okay?"
Rainbow cried:
,,No! Im not okay! Nopony understands me! Even when I was younger pony!"
And later she feels the scary feeling in her chest and she starts floating. Around her was a black loop. Inside loop from Rainbow eyes started falling tears from her eyes only because she was feeling pain. But not for long. Later Rainbow body changed into black,alicorn body with rainbow floating mane and fangs. When the loop dissapeared Nightmare Dash let's out an evil laugh.
END OFF PART 1.
Practice makes perfect
I think the beginning of 1st part will be story about the 3rd Princess Celestia sister- Princess Of Weather what having the power of sun and moon,plus she having her own power called weather when one of sisters being sick. She helped too the other ponies with her own power. But one day Princess Celestia gets mad at her older than her sister.
,,You doesn't supposed to be my sister! Y-you ruined anything! You can't work even with all those weather!"
,,Well. See you about it. Celestia."
And then she changes into Weatherbreaker. (Nightmare Dash). Princess Celestia doesn't know what she needs to do. And then. She got an idea. Banished her too. To. Ponyville. Princess Of Weather planned the revenge for Celestia. But. How? Easy. She used her horn and she did a spell what can change her in pegasi or unicorn or even earth pony! She chooses to being pegasi and she changes into Rainbow Dash.
After story Twilight gasps in shock and she asks Rainbow does she's older than Celestia. Rainbow started thinking about lie and she said there's so many ponies what that pony can change and not only she can be. But Rainbow after going home started thinking does that story is real. In next day she founds in her shelf an strange amulet what only in Day Of Sun she can change into Weatherbreaker. (Action in 1st part is day before Day Of Sun and the moment with Rainbow Dash transformation.)
Later she goes to Twilight to asks what is this. But Twilight started again asking her about Princess Of Weather and does she is her. Rainbow can't handle this anymore and she said:
,,Guh! Okay! You got me! I am that Princess Of Weather! Happy now?!"
Twilight gasped in shock and she asks does she dangerous. Princess Of Weather in Rainbow Dash body said:
,,No. Well. Maybe a little bit."
Later Twilight did the same happy face what she did in s7e25 or 26 and she did the spell who can she see in Rainbow Dash body. But the visuals was more crazy than she expected. Princess Of Weather was a white alicorn princess with wavy rainbow mane. Her cutie mark was sun with rain. Anywho. Visuals wasn't longer than few minutes because Rainbow Dash runned away crying and saying:
,,Y-you will never understand me! Even if I am the real Princess Of Weather! S-so. Just leave me alone!"
Later in Everfree Forests after crying short scene she feels a strange feeling in her chest. Weatherbreaker mumbled to herself:
,,Hm. It's getting dark. Better gets outta here until the festival in Ponyville town hall started. And I. Having the special show for everypony."
And she flies away. Later Rainbow heard Rarity voice saying:
,,And now! Let's clap your hooves for Princess Celestia!"
Rainbow screamed:
,,No! You can't do this!"
Other ponies looked at her. Twilight asked:
,,Princess Of... I mean. Rainbow Dash? Are you okay?"
Rainbow cried:
,,No! Im not okay! Nopony understands me! Even when I was younger pony!"
And later she feels the scary feeling in her chest and she starts floating. Around her was a black loop. Inside loop from Rainbow eyes started falling tears from her eyes only because she was feeling pain. But not for long. Later Rainbow body changed into black,alicorn body with rainbow floating mane and fangs. When the loop dissapeared Nightmare Dash let's out an evil laugh.
END OF