This story is a sequel to The Empress and the Goose-Girl
Rarity is beautiful, elegant, talented. She has to be, to survive as Seneschal to her Dark Majesty. But now a newcomer arrives at the court of the Empress Nightmare Moon, and she threatens to unsettle everything Rarity has worked so hard to build.
Technically a sequel to The Empress and the Goose Girl, but works fine as a standalone if you don't want to read that. TW for some violence and blood, but it's not massively graphic. Written as a gift for the lovely Zontan, as part of an anthology.