Morsmordre

Morsmordre is the incantation of the spell used to conjure the Dark Mark, the skull-and-serpent sign used by Voldemort and his Death Eaters. It is very likely that Voldemort invented this piece of magic and taught it to his followers. Only Death Eaters and Lord Voldemort know the correct way to cast the spell, although Horace Slughorn is implied to know how to cast the spell as well. Death Eaters often cast it after committing a murder, placing the Dark Mark in the sky over the location of the victim's corpse. It is against the law to cast this curse.

Before 1991
"...Just picture coming home, and finding the Dark Mark hovering over your house, and knowing what you're about to find inside."

- Arthur Weasley after the Dark Mark was cast at the Qudditch World Cup

Whenever the Death Eaters killed a person or persons they cast the Dark Mark over the place where they were killed.

1994
Barty Crouch Jr used this spell to set off the Dark Mark during the 1994 Quidditch World Cup using Harry Potter's wand, in order to scare the Death Eaters who were disloyal to Voldemort. At first, the house-elf Winky was blamed for the act until the truth was revealed by Crouch himself under the influence of Veritaserum.

1996
Igor Karkaroff was killed in 1996 by Death Eaters for betraying Lord Voldemort, and the Dark Mark was cast over the shack in which his body was found.

1997
It was cast over Hogwarts' Astronomy Tower by Gibbon when the Death Eaters invaded the castle in 1997, in order to lure Albus Dumbledore to the tower so that Draco Malfoy could kill him. However, it was Severus Snape who committed the murder, and Dumbledore's corpse was described to have fallen off the tower under the Mark's eerie green light like a great rag doll.

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. In Norwegian, Morsmordere directly translated means "mother killers".

Behind the scenes

 * In the film version of Harry Potter and the Goblet of Fire, the Dark Mark only appears greenish when it was cast at the Quidditch World Cup. When cast later in the same film after Voldemort's resurrection, along with its appearances in the film version of Harry Potter and the Half-Blood Prince, it appears to be formed out of dark grey clouds.
 * In the book version of Harry Potter and the Goblet of Fire, the Dark Mark was cast over the Quidditch World Cup by Bartemius Crouch Jr. in order to strike fear at disloyal Death Eaters during their torturing of the Roberts family. However, in the film version, the Mark was cast after the Death Eaters destroyed the campsite, as a sign of the Death Eaters' works.
 * In the book version of Harry Potter and the Half-Blood Prince, the Dark Mark was cast over the Astronomy Tower by Gibbon in order to lure Dumbledore there. However, in the film version, the Mark was cast by Bellatrix Lestrange in celebration after Dumbledore was killed. Furthermore, in the film the Mark was dispelled by the mourning residents of Hogwarts as a sign of respect for the late headmaster.

Appearances

 * Harry Potter and the Goblet of Fire
 * Harry Potter and the Goblet of Fire (film)
 * Harry Potter and the Goblet of Fire (video game)
 * Harry Potter and the Half-Blood Prince
 * Harry Potter and the Half-Blood Prince (film)