@JP
 
She is a Nyx. A very bizarre alt form.  
This story arc concept revolve around 4 alternate reality Nyxes coming together to fight Nemesis. A reality hopping alicorn out to hunt down other alternate reality Nyxes, kill them and consume their powers while at the same time laying that reality Equestria to waste.  
Three anomaly Nyxes are recruited who posses special abilities unique to them.  
These abilities are unlocked once they push their powers beyond standard alicorn levels.  
However the powers are unstable if pushed too far and also threatens to destabilize the psychology of the Nyx
 
Nyx Petite Pie.  
Reality Bending. Emblem of Dark Magic and Earth Ponies.  
Alternate name “Nightmare Abyss”  
For her ability to turn reality into a nightmare.  
Threat. When turned into Nightmare abyss there is an internal struggle between the host Petite Nyx and the nightmare powers.  
At it’s worse she’ll lose control, her personality is swallowed up by the nightmare and turn the whole of Equestria something akin to Silent Hill that would put Discord to shame.
 
Nyx Sparkle  
Energy (lots of it). Emblem of Neutral Magic and Pegasus Ponies.  
Alternate name “Zeta Flare”  
Zeta is a blue super giant star (the color her coat takes when ascending beyond alicorn power levels) which means that this is a Nyx that can easily outshine Celestia in terms of heat and light.  
For her ability to blow everything away and turn mountains into Lava.  
Threat. While at this state her body who acts as a vessel slowly dies off (how fast it dies depends on her training level) If she dies however she’ll go Super Nova.
 
Dawn Apple  
Healing. Emblem of light Magic and Unicorn Ponies.  
Her biggest problem is total lack of experience being raised by Earth Ponies who adopted her. She also has the introvert personality of Fluttershy.  
Her blood has healing properties for poisons to flesh trauma.  
Unfortunately Dawn is a Heamaphobe (irrational fear of blood).  
But her power at its most potent she can terraform a dead planet into Eden.  
Being light magic there are no negative side effect to her powers

I’m quite certain this was at one point Nyx. This artist has drawn Nyx many times before.