A patronus is a guardian against a Dementor, and can be summoned by using the charm, Expecto Patronum. However, the incantation isn't enough. To summon a patronus, you also need to think of a very happy thought or memory. The happier it is, the more powerful your patronus. A patronus is either corporeal or non-corporeal. An non-corporeal patronus takes the form of a silvery shield. A corporeal patronus takes the form of an animal, and each is unique to the witch or wizard who conjured it.