DayBreaker

I'm positive that while Nightmare Moon represents Luna's feelings of neglect and jealousy the Daybreaker represents Celestia's regrets and rage at 1000+ years of losses and troubles, that she could have prevented if she'd chosen to be more forceful and domineering with her subjects.

Any hint of noncompliance triggers powerful anger and punishment.  The children have used up all their time-outs and suspended privileges, exhausting mommy in the process.  She's got to get through to them now and if pain and terror is the only way left, so be it.