Protego horribilis

Protego Horribilis pro-TAY-goh horr-uh-BIL-lis (IPA: [pɹəʊ.'teɪ.gəʊ ˌhɔɹ.ɚ.ˈbɪ.lɪs])

A shield charm against the darkest magic.

Uses
Cast by Professor Flitwick in an attempt to strengthen the castle's defences in Harry Potter and the Deathly Hallows

Latin Protego, "I protect", and Horribilis, "horrible, frightful, dreadful".