Twenty-Five Over Zero (25/0) is what I saw happening in episode 125, "Party of One." I realize many are tired of such stories, but I hope you can enjoy it regardless.
After her friends express disinterest in her party, Pinkie loses herself, swapping physical control with a dark personality within her. Furious at the turn of events, and excited to be free, this externalized inner-evil gathers all her former friends, ready to put her plan into action, tormenting them physically and destroying them mentally. No one may leave until the party is over.