After her forced reformation which altered her soul, the old Sunset Shimmer gets a second chance due to Nightmare Night enveloping the world. She now seeks revenge on everyone with the help of the Dazzlings.

The first targets? Her former inner circle! Rarity proves easy to lure away, as she pretends to be the kinder Sunset she knew, and Spike has been turned into a wild beast with primal needs only his crush can sate...

This is set in the same continuity as The Fourth Siren, but it isn't necessary to read that first.

https://www.fimfiction.net/group/214700/dirty-little-secrets-dirty-little-contest