Riddikulus

Riddikulus is the only known reliable defense against a Boggart — the shape-shifting magical creature that lives in dark cupboards and old houses, taking on the form of whatever a witness most fears the moment it sees them. The charm doesn't kill the Boggart; nothing standard does. What Riddikulus does is force the creature into a form it cannot menace anyone with, and laughter from the surrounding witnesses is what actually drives the Boggart away.

The classic introduction comes in Remus Lupin's third-year Defence Against the Dark Arts class — one of the warmest single chapters in the entire series. Lupin lines the students up in front of an old wardrobe containing a Boggart and walks each one through their fear's transformation. Neville's grandmother-shaped Boggart Snape becomes a Snape in Neville's grandmother's clothing; Parvati's mummy unwraps and falls over; Ron's giant spider gets roller-skates and slides off uselessly. The lesson is partly about the charm and partly about the larger truth that fear, faced and made absurd, generally loses most of its grip.

The charm has two requirements that are easy to state and harder to do under pressure. The caster must clearly picture the ridiculous form the Boggart should take, and must genuinely find that form funny. A reluctant or grim cast produces a weak transformation that the Boggart can recover from; a confident, mocking cast produces a Boggart that is no longer dangerous to anyone in the room. Multiple casters laughing together compound the effect, which is why DADA teachers traditionally introduce the charm in front of a class.

Boggarts have one specific weakness worth noting. Faced with multiple people, a Boggart can become confused about whose fear to embody, sometimes splitting into half-formed transitions that no one in particular is afraid of. Lupin uses this in his first-year demonstration, having the class crowd in front of the wardrobe at once. The Boggart's confusion is part of what makes Riddikulus work.