The Patronus Charm (Expecto Patronum) was a charm that conjured a translucent silver mist, a bright shadowy beast that protected the caster from the haunting effects of the presence of a Dementor or Lethifold. It is highly complicated, advanced magic, well beyond the capabilities of even a seventeen-year old. To cast it, one must use the happiest memory they can think of. Strong emotions such as love can change one's Patronus shape