Spells
Stupefy
Stupefy is the Stunning Spell. It knocks a person unconscious in a flash of red light. Duelling clubs, Ministry squads, and Death Eaters all use it. Several Stunners at once can seriously injure the target, which is how the Order members discuss the risk in the Department of Mysteries.
In the books
Harry Potter and the Goblet of FireChapter: The Third Task
Stunning spells are part of the maze. The incantation is in heavy use from this book onward. This page does not claim an earlier first line.