Twilight Sparkle fucks and then stabs her brother to death so that his blood can be used to allow Cadance to turn into a sex demon and slaughter dozens more ponies so she can regain her youth.

Written for the Spoopy October Incest Contest in a single day. I know it's going to get downvoted to all hell because it's contains gore, but they had so few entries and I felt that I needed to crash in and terrify everyone.