Rainbow Dash lost control of her vampire power five hundred years ago, and she's never been the same mare since.
Numerous ponies find themselves caught in the web of this vindictive killer's game.
Sweetie Belle is one such pony.
Twilight Sparkle was Rainbow's partner for life until that day. A vampire as well, she regained control of her power back then but became the eternal target of Rainbow's vengeance thereafter.
Crimson, the vampire who turned Rainbow Dash and Twilight Sparkle, didn't know Rainbow would turn on Twilight, but is otherwise proud of her being his legacy.