This story is a sequel to Applemania
Fluttershy's batty side doesn't bother her much anymore, but now Pinkie's gotten it in her head that Fluttershy should drink somepony's blood. Being the dutiful girlfriend she is, Pinkie offers her own, and with Applejack's help manages to get Fluttershy to agree to try it out.
Content warning: I don't shy away from describing things in this story. There will be blood. Quite a bit of it.