Also, to all reading, please read Angron: Slave of Nuceria by Ian St-Martin. Props to him, that's where I got the contest from as that's canonically what happened to the young Primach Angron. I couldn't have come up with a more twisted way to torture somebody than a death race to the top of a pyramid.

angron suiting fit for fluttershy, before the nails he was very empathetic towards others, and with the nails would explain her sudden rage and anger.
Personally i would prefer the night haunter but eh this is good loved the book.

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Angron's empathy is what drove me towards choosing Fluttershy to be the Angron of the Ponyhammer universe she witnessed through the orb. The Nighthaunter was my second choice due to the whole Flutterbat escapade. Fret not, The Nighthaunter and other characters will start to leak into the dreams of others. Going for a slow build up with this story, which is why it's taken 3 chapters to get to the reveal of a mere fragment of what Fluttershy witnessed while comatose.

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not a bad choice
i like the night haunter because he is the complete opposite of her and im curiious if her kind nature would temper the crueler nature of the night haunter if she lived through his life and also i like the night lords.
Angron well his empathy early on and its hella easy to explain her anger and rage due to the nails.
plus adds more into the tragic backstory especially with fluttershy as a character
oh the courage and honor speech i can see it now

Well. I'm not sure what I was expecting, but it certainly wasn't that.
Now I'm wondering who you're going to match up with each primarch. You've already implied Twilight and Magnus, but now I'm wondering who you'll pick for the rest of them, assuming you're going for all traitors. In the meantime, have some predictions:

Most people connect Fulgrim and Rarity for being vain, flamboyant perfectionists, but I personally get Applejack vibes from him; both have some experience with manual labour and running a business, and they're both fiercely loyal to those they view as family, with their biggest weaknesses being their own pride (SSCS6000) and desire to live up to others' expectations turning into an obsession (The last roundup).

Pinkie's smarter than she looks and keeps tabs on everything (Party pooped), and is more insecure than she lets on (Party of one), which sounds like either Alpharius or Perturabo.

Rainbow's impulsive and kinda self-centered, but has a strong sense of justice (Over a barrel) that she's willing to break if she feels she needs to (Tanks for the memories). Definite Konrad vibes.

Rarity... eh. She's intelligent with an eye for detail, but bases a lot of her self worth on how others view her (Suited for success), so maybe Perturabo?

Either way, eagerly looking forward to the rest of it!

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i dont think she would be a night haunter, if the orb gave them all visions of the different primarchs, it had to be one with a decent childhood, out of all of them Fluttershy is the only one to have visions of death and gore, the night haunter was practically all that. time will tell but i dont see the night haunter a good fit for rainbow, that being said i am a firm believer if you threw any of these characters onto nostramo they would be something akin to the night haunter if they wanted to survive

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Conrad Curse was already insane. Angron is a perfect choise for Fluttershy; like you said he was an empath and he had EMPATY for others before the nails. Angron became what he is now because the High Riders and Nuceria broke him . He tried to save his Gladiators friends but when he couldn't, he gave them a clean death (when he wasn't occupied fighting for his life against monsters, mutants and Nail broken men).

Nucerian Flutteshy for the win.