Throughout what scant is shown of her long and storied history, Celestia has faced a large number of foes, among which stand tyrants, gods, and monsters. Each represented a different face of the force of Evil as Equestria knows it, and each sought for one reason or another a goal that would bring suffering and even death to ponies, and possibly all life in the world. Standing against these forces and knowing the threat they represented, Celestia displayed an array of emotions:

Against Nightmare Moon, her own sister Luna twisted by loneliness into violent insanity, Celestia expressed grief and resignation in her task to save the world from her.

Against Discord, the cruel and apathetic god of chaos, Celestia was stern. Her demeanor was that of a strict parent disciplining a rampant child.

King Sombra, the tyrannical oppressor of the Crystal Empire, was met with grim resolve. It was as though she were bracing herself for a necessary evil.

When faced by Queen Chrysalis, the invasive monarch of the Changeling horde, Celestia countered with the cold determination to protect her subjects at any cost.

Lord Tirek was met initially with great fear, but even that did not become the kind of hate necessary to fuel Dark Magic, as was evidenced by its absence from the power she granted to Princess Twilight.

In short, despite their great power, cruelty both to her and those dear to her, and threat they each represented to her and her subjects, somehow none of these beings have managed to inspire the hatred Celestia demonstrated being capable of. Which means that there are other villains not yet encountered, at least one of which is likely a blemish on the Sun Princess's past so dark that it is unfit for a children's show. This is the story of that villain: another alicorn Princess who only wants what's best for her subjects. The problem, of course, lies in just what she believes that is.

(I feel it bears mentioning that the "sex" and "gore" tags are intended for the second chapter, and/or the third, and that if you came here looking explicitly for those, you are probably going to regret doing so once I provide them. This story is intended to be horrific, not gratifying.)