Periculum

Periculum was a spell that allowed the caster to send out red sparks from their wand. The sparks were firework-like in appearance, and had a vast range, shooting up to a great height, then hovering in the spot where the caster aimed. Much like a Muggle flare gun, the spell was used to signal that one was in danger, and thus in need of help.

Known uses

 * In 1992, while searching for an injured unicorn Rubeus Hagrid ordered Harry Potter, Hermione Granger, Neville Longbottom, and Draco Malfoy to send red sparks in the air if they were in trouble and green sparks if they found the wounded animal.
 * Minerva McGonagall instructed the four champions to send red sparks into the air if they needed to be rescued prior to the Third Task of the Triwizard Tournament in 1995. Later on, at the urging of Harry Potter, Cedric Diggory used this spell to send red sparks into the sky to call the Hogwarts professors to rescue Viktor Krum, who had been Stunned by Harry after he attacked Cedric whilst under the Imperius Curse set upon him by Alastor Moody (being impersonated by Barty Crouch Jr.).
 * The Order of the Phoenix may have used this spell in 1995 as the signal for the Advance Guard escorting Harry to 12 Grimmauld Place.

Etymology
Periculum is Latin for "danger".

Appearances

 * Harry Potter and the Philosopher's Stone
 * Harry Potter and the Goblet of Fire
 * Harry Potter and the Goblet of Fire (film)
 * Harry Potter and the Order of the Phoenix