The Calamity was a time of great change. Spirits can handle gradual change, however sudden change can cause them to 'break', especially when it occurs within their domain. Given the nocturnal nature of the monster swarms, Luna was hit hardest by the Calamity's effects.
I have a different idea for what Luna's gradual transformation into Nightmare Moon would look like if I drew this again.
Program: Clip Studio Paint
Tablet: Artisul D16
Time taken: 3 hours