Expecto Patronum

The Patronus Charm is perhaps the most famous and one of the most powerful defensive charms known to wizardkind. It is an immensely complicated and an extremely difficult spell, that evokes a partially-tangible positive energy force known as a Patronus or spirit guardian. It is the primary protection against Dementors, against which there is no other defense. Because this is the one spell that will scatter an army of Dementors, it is quite unbeatable to them. However, using this on any other creature will prove to be useless.