- " [...] although he could barely see out of it, he pointed his wand through the smashed window and started muttering incantations of great complexity. Harry heard a weird rushing noise, as though Flitwick had unleashed the power of the wind into the grounds."
- —Flitwick casts enchantments such as this one over the school[src]
Protego horribilis is the incantation to a stronger version of the Shield Charm which protects a very large area against highly Dark Magic. It causes anything within the ranges of Dark Magic, except the Killing Curse, to rebound off of the shield or dissipate. The only known way to deflect the Killing Curse is sacrificial protection, of which someone dies to save someone else, using the magic of love.
This spell was cast by Charms Master Filius Flitwick to protect the boundaries of Hogwarts Castle from the Death Eater forces in the late hours of May 1, 1998, prior to the Battle of Hogwarts that started at midnight the next day.
Latin Protego, "I protect", and Horribilis, "horrible , frightful, dreadful"; altogether the charm means "I protect against the horrible."