This is a story of time paradoxes, distortions and fate incarnate.
A particular Sunset Shimmer has successfully destroyed the world at some point in time and inflicted a fate worse than death to her enemies. That world will eventually pulled itself back together and civilizations will have expanded back to the same point where it was before Sunset Shimmer originally destroyed it.

A new world does not matter, for she will appear and do it again.

Time travelers have failed.
Heroes have failed.
Monsters have failed.

This vindictive world destruction slowly spreads into all other futures and a few distant universes. The Rune Witch is a drain, sucking all similar Equestrias to a hellish fate. Anyone who is anybody important, through visions of the future or insane immortal entities living beyond the terrible destruction of their worlds is aware of it. Aware of Her. Nothing can stop the abomination. Not words or even mere death.

They have resorted to the final option of rooting out the witch.

Sunset Shimmer shall not be allowed to exist.