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(Story Concept)
First person fic. Rainbow Dash dies, an unhinged Twilight Sparkle uses a mix of necromancy, conjuration and changeling magic in an attempt to create a new body for her. Enter the protagonist through the mirror pool with ALLEGEDLY no memories related to it's own past, and a patchwork of mostly useless knowledge.

Primarily Related Tags
-adventure, dark, comedy, everything else (except human and eqg)

Rating
Teen or Mature

Notes:
1) The protagonist claims no known background, no species, nothing that typically ruins a HiE fic.
2) The new Rainbow tends to make jokes and references alien to ponykind
3) Snippets of random knowledge make the main character more of a wildcard than ever, altering between moments of brilliance and stupidity surpassing even the original Rainbow Dash.
4)In theory only a willing creature's spirit could be summoned this way
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6383503
done "right"? that's HIGHLY debatable, especially what you might consider "wrong". But disregarding that, what exactly is your story? it seems like you're combining three stories: 1. Twi bring RD back from the dead, 2. a Deadpool/Pinkie Pie-like character interacting with the straight-characters, and 3. modern world protagonist (with amnesia, not uncommon) being taken to a magical world

The only thing I can guess is that it's something along the lines of the soul not being whole, like pouring a cup of water into the ocean, then trying to fill the cup back up with the water you just poured out (I forget the exact quote) ? but if that's the case, then what does the mirror pool have to do with anything? Unless the plot point of RD being brought back to life has ABSOLUTELY NOTHING to do with "the protagonist" entering Equestria...

am I the only one confused right now?

6383848
The idea is that Twilight's attempt fails in the usual fashion. RD is still dead, there is somepony else who looks like her.
You can't exactly retain your memories if you leave your brain behind, especially if the spell was modified to bring someone else, so we get rid of the human identity (partially) and render most knowledge this creature would otherwise bring with itself almost completely useless.
The main character CLAIMS amnesia. That is a half-truth, what they remember (and how much) is up to the writer, but some of it is out of context, a constant cause of failure. On the upside, it also allows them to be selective with anything they share with others.

It is technically just a HiE where the 'not-human' character has no reason to repeatedly reference itself in more than a few words or rant about humanity (with a side of reverse Mary Sue)

Originally it was a Luna fic idea where the main character is forced to help Daybreaker, but I couldn't come up with a good enough excuse for that.
Now that you mentioned „Pinkie Pool” ...