Caterwauling Charm

"The air was rent by a scream... It tore at every nerve in Harry's body, and he knew immediately that their appearance had caused it."

- Harry Potter, Ron Weasley, and Hermione Granger setting off the on Hogsmeade The Caterwauling Charm (incantation unknown) causes any unauthorized person entering the perimeter of a certain area to set off an "alarm" which makes a high-pitched scream.

History
"They were ready for us. They set up that spell to tell them we'd come. I reckon they've done something to keep us here, trap us —"

- Harry Potter on the Death Eaters' use of the charm

During the height of the Second Wizarding War, a Caterwauling Charm was placed on the grounds of Hogsmeade, which would be activated if anyone went on the village streets after a certain time. This curfew was put in place so that the Death Eaters could be instantly aware if an intruder (they were particularly interested in Harry Potter) entered the village past the curfew, immediately setting off the high pitched wail of the charm.

As it happened, Harry and his friends Hermione Granger and Ron Weasley did indeed accidentally set off a Caterwauling Charm when they Apparated into Hogsmeade on 1 May, 1998. This alerted the Death Eaters in the village to their presence, although the trio managed to avoid being captured with the help of Aberforth Dumbledore, who convinced the Death Eaters he had set off the charm by putting his cat out of his house, and was able to pass off his goat-shaped Patronus for the stag-shaped one that Harry was forced to conjure (under his Invisibility Cloak) in order to ward off the Dementors that the Death Eaters summoned to find him and his friends.

Behind the scenes

 * It is possible this is another form of the Intruder Charm, which is more suitable usage for a single household and less loud, as Horace Slughorn could not hear it while in the bath.
 * It's basically the wizard version of a military base alarm system.

Appearances

 * Harry Potter and the Deathly Hallows
 * Harry Potter and the Deathly Hallows: Part 2
 * Harry Potter and the Deathly Hallows: Part 2 (video game)
 * LEGO Harry Potter: Years 5-7