This story is a sequel to Biting Buddies
After research into Rot Wolves, Twilight discovers Cadence was cursed by the Rot Wolf Hexirit many years ago and it has taken effect on Flurry Heart.
When the curse is removed, it reverts Flurry Heart into a regular unicorn. Twilight tries to turn her back into an alicorn before her brother finds out.
This has been reviewed by me HERE.