Riddikulus

Riddikulus is a charm used to repel Boggarts, magical creatures that take the form of one's greatest fear. The spell forces the Boggart to transform into a comical or harmless version of itself. To cast it effectively, the caster must think of something funny or absurd to turn their fear into. Professor Lupin taught the students this spell as a practical defense against Boggarts.

Harry Potter and the Deadly Hollow