After the murder of Rainbow Dash, her ghost feels as though she has not finished her business in the normal world. She seeks revenge on her so called friend Pinkie Pie, and to get her found out before more ponies are hurt.

BTW!

Art cover used from DeviantART user: aisu-isme. Credit to her! You should check out her gallery too.

P.S.
This has been edited with some new ideas.