The Patronus Charm is one of the most ancient of charms and appears in many accounts of early magic. In spite of a long association with those fighting for lofty or noble causes, the Patronus is not unknown among Dark wizards.

While there is a widespread and justified belief that a wizard who is not pure of heart cannot produce a successful Patronus, a rare few witches and wizards of questionable morals have succeeded in producing the Charm. It may be that a true and confident belief in the rightness of one’s actions can supply the necessary happiness.

No reliable system for predicting the form of an individual’s Patronus has ever been found, although the great eighteenth-century researcher of Charms, Professor Catullus Spangle, set forth certain principles that are widely accepted as true.

The Patronus, asserted Spangle, represents that which is hidden, unknown but necessary within the personality. "For it is evident a human confronted with inhuman evil must draw upon resources he or she may never have needed, and the Patronus is the awakened secret self that lies dormant until needed, but which must now be brought to light."

Here, says Spangle, is the explanation for the appearance of Patronuses in forms that their casters might not expect, for which they have never felt a particular affinity, or even recognize.

"Such a Patronus is an indicator of obsession or eccentricity. Here is a wizard who may not be able to hide their essential self in common life, who may, indeed, parade tendencies that others might prefer to conceal. Whatever the form of their Patronus, you would be well-advised to show respect, and occasionally caution, towards a witch or wizard who produces the Patronus of their choice.’