Drugged

Cupcakes is a dark fan fiction set in the My Little Pony: Friendship Is Magic universe. It depicts the character Pinkie Pie as a serial killer (though still keeping her in-universe bright and cheery demeanor) who drugs her friend Rainbow Dash, drags her to a hidden cellar and proceeds to slowly and brutally mutilate her to death. The extremely graphic nature of this fic has made it rather infamous within the MLP fandom.