Morsmordre

"Then he realised it was a colossal skull, comprised of what looked like emerald stars, with a serpent protruding from its mouth like a tongue. As they watched, it rose higher and higher, blazing in a haze of greenish smoke, etched against the black sky like a new constellation."

- The Dark Mark

Morsmordre is the incantation of a spell used to conjure the Dark Mark, the skull-and-serpent sign used by Lord Voldemort and his Death Eaters. The Death Eaters cast their mark over the homes of anyone they killed.

Practicioners

 * Bartemius Crouch Jr.
 * Bellatrix Lestrange
 * Horace Slughorn (possibly)
 * Gibbon

First Wizarding War
In the First Wizarding War, the Morsmordre curse was somewhat of a signature move for Death Eaters. Upon committing murder, they would cast it over the locations as a declaration of their crimes. This led the wizarding community to fear the mark's appearance, as it would usually mean death is found when it is present.

After Voldemort's first defeat, the mark became absent when Death Eaters were either rounded up or evaded justice, and due to very few wizards knew how to cast it, almost exclusive to Death Eaters. Because of its affiliation, the casting of this curse is considered illegal.

1994
Barty Crouch Jr. witnessed Death Eaters torturing the Roberts family for entertainment, which awakened him from the Imperius Curse his father placed on him. He took Harry Potter's wand, which was sticking out of the boy's pocket in front of him, and cast the curse to set off the Dark Mark during the 1994 Quidditch World Cup, in order to scare the Death Eaters there for being disloyal to Voldemort. This caused mass panic to both the audience and Death Eaters alike, causing the former to run around in hysteria and the latter to Disapparate in fear of facing their master's wrath.

At first, the house-elf Winky was blamed for the act due to the culprit wand was found in her possession and the record showed it had performed the curse. Much later, the truth was revealed by Crouch himself under the influence of Veritaserum. The Dark Mark's appearance over the campsite was a huge scandal amongst the British magical community, and many citizens filed various complaints to the Ministry.

Voldemort would later comment on Lucius Malfoy fleeing at the sight of the mark as a sign of disloyalty when Lucius was hastily making poor excuses.

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 French mort, "death", and mordre, "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 "mothers killers".

It may also be a reference to the word "mord", which means "murder" in Icelandic, German, Norwegian, Danish Polish and Swedish, since the spell is cast after someone has been murdered, and sounds like "mass murder".

Behind the scenes

 * Despite the film adaptions showing that the caster simply has to point the wand upwards and speak the incantation to launch the mark, in Harry Potter: Spells shows the hand movement required.
 * 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 Barty Crouch Jr. with Harry's wand in order to strike fear at disloyal Death Eaters during their torturing of the Roberts family. However, in the film adaption, the Mark was cast by his own wand after the Death Eaters destroyed the campsite, as a sign of the Death Eaters' works.
 * In the film adaption of Harry Potter and the Half-Blood Prince, the Dark Mark was cast after the Death Eaters attacked Diagon Alley and destroyed the Millennium Bridge, with it towering over London.
 * 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 adaption, the Mark was non-verbally 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.
 * This is the only spell that Barty Crouch Jr. casts onscreen, in his true form, before becoming Alastor Moody by means of Polyjuice Potion.

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)
 * Harry Potter and the Deathly Hallows: Part 1
 * Harry Potter and the Deathly Hallows: Part 2
 * Harry Potter and the Forbidden Journey
 * Harry Potter: Spells
 * Pottermore