Morsmordre

Morsmordre is the incantation of the spell used to conjure the Dark Mark. Only Death Eaters and Lord Voldemort know the correct way to cast the spell, which was likely invented by the latter, although Horace Slughorn is implied to know how to cast the spell as well. Death Eaters often cast it after committing a murder. It is said that the Dark Mark is placed over a house in which the Death Eaters murder.

1994
Barty Crouch Jr used this spell to set off the Dark Mark during the 1994 Quidditch World Cup using Harry Potter's wand. At first, the house-elf Winky was blamed for the act until the truth was revealed by Crouch himself under the influence of Veritaserum.
 * The Death Eater or Death Eaters who killed Igor Karkaroff in 1996 for betraying them cast Morsmordre over the shack they killed him in.
 * It was cast over Hogwarts' Astronomy Tower the night Albus Dumbledore was killed by Severus Snape and Death Eaters invaded the castle in 1997.

Etymology
The incantation of this spell appears to be derived from the Latin mors, "death", and mordere, "to bite". This alludes to the name of Voldemort's followers, the Death Eaters. The English word murder could also contribute.

Appearances

 * Harry Potter and the Goblet of Fire
 * Harry Potter and the Half-Blood Prince