Using ancient magic, Trixie summons the Pooka, the legendary spirit of the wild, to help her get revenge on Twilight Sparkle. As usual, things don't go as planned, and she learns just what happens when you mess with old magic. Now the Pooka is loose, and is determined to reestablish himself as the rightful ruler of the Night.
To be honest, I think Trixie got lucky. A (possibly) malevolent spirit that's been trapped tends to have a very active imagination when it dishes out punishments.