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Rocket to Insanity is a Grimdark fanfiction widely regarded as the sequel to Cupcakes. It focuses around Rainbow Dash having nightmares about Cupcakes every night, going partially insane as a result, and subsequently killing Pinkie Pie and turning into the Church Guy from Left 4 Dead. Much like Cupcakes, it is very graphic, but has a lot less disembowelment, as Rainbow Dash prefers to just kill Pinkie rather than torture her. Thus, as it was said in the Cupcakes article, eventually Luna is going to have another temper tantrum and become Nightmare Moon and with the main six down an Element conquer Equestria.
An alternate ending was written in which Pinkie Pie was not killed, merely scarred, and Rainbow Dash didn't go insane (this alternate ending is actually longer than the story itself).