Twilight's fantasies are growing more kinky and taboo nature and so, concerned by this, she seeks out Rarity's advice.
What she gets is far more than advice, but she's far from upset with the turn of events.

Twilight x Rarity fic with lots of bondage and Sadomasochistic themes. If that's not your thing then back away slowly. If it is then hopefully this will rouse your interest.