Patronus

A Patronus is a spell cast by a witch or wizard to deflect dementors. A proper Patronus takes on a shape reflecting the caster's inner self and often has the same or a similiar shape to the person's Animagus.

The spell is performed by focusing on happy thoughts while uttering the words Expecto Patronum. A wizard who has not yet learned to conjure a full Patronus might still be able to produce a shield that will hold off the dementors for at least a while. When producing a full Patronus, it can drive away all dementors in the area.

The spell is seen as one of the most powerful spells ever created.

The usage of a Patronus is first seen in the book Return of Sauron; Returned as Sonya casts the spell to drive off attacking Ringwraiths, who also seem to be affected by the power in the spell but cannot be hold back for long by it, unlike dementors. Sonya's Patronus is seen as an owl, which is not the animal she can shift into, but the animal-shape of her master in Dweomer - Aderyn. Some do believe, though, that the owl is Sonya's second form, but this has never been confirmed.

The spell is later used by Remus Lupin (dog), Freedom (lion), Harry Potter (stag) and Sandra Roberts (phoenix) during different stages of the story.