Cave Inimicum

From UnknowableWiki

Cave inimicum is one of the spells that Hermione Granger uses to protect the tent that she shares with Ron and Harry while on the run from Death Eaters. She first casts it in the woods where the Quidditch World Cup was held (DH14), and presumably does the same in the following months, in several other forests (DH15, DH18, DH19, DH22). The charm, cast with a skyward flourish, probably offers some sort of warning when enemies approach (DH14).

Hermione also uses this spell to protect the Trio soon after they Disapparate from Xenophilius Lovegood's house (DH22).