The Patronus charm is very advanced magic in the wizarding world. To summon a Patronus, the witch or wizard must think of a happy thought and say 'Expecto Patronum'. The charm results in a silvery cloud (or shape, depending on the person casting the charm). A Patronus can drive away dark creatures, such as a Dementor or a Lethifold (a creature described in 'Fantastic Beasts and Where to Find Them').
Harry learns to cast the Patronus charm in 'Prisoner of Azkaban' to drive off the Dementors which cause him to hear the dying screams of his mother, as Lily died saving her son from Lord Voldemort's wrath.
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