Renders unlocking spells useless
The Anti-Alohomora Charm (incantation unknown) was an anti-unlocking spell invented by Blagdon Blay sometime in the early 17th century. His house had been broken into nineteen times through use of the Unlocking Charm, and so he invented this spell to counteract it.
Behind the scenes
- Harry Potter suspected that Dolores Umbridge, then Defence Against the Dark Arts professor, locked her office to prevent the Unlocking Charm from working, possibly referring to this charm.