A signature spell (or signature move) is the spell most commonly cast by a witch or wizard, especially when used in the context of duelling. Even though the spell may be the one the caster is best at, its predictability can make its user vulnerable.
According to Remus Lupin in 1997, Lord Voldemort's Death Eaters thought Harry Potter's signature spell was the Disarming Charm, and he urged Harry not to allow that to be the case. Despite this, Harry continued to make good use of the charm, and ultimately used it to defeat Voldemort.
The Killing Curse was thought by most of the wizarding world to be Voldemort's signature spell, due to his frequent and indiscriminate usage of it; he certainly massacred enough to create an army of Inferi. He also invented the Morsmordre curse for his subordinates, to which it became somewhat of a signature as they usually cast it over locations where they commit murder.
Behind the scenes
- The Full Body-Bind Curse seems to be the signature move of most if not all Pottermore users, as it is the most popular out of all (not without reason, as it is the most effective spell when Duelling, if properly cast); Furnunculus is the least popular.
- Harry Potter and the Deathly Hallows (First appearance) (Mentioned only)