Should the Dark Mark appear over any dwelling place or other building, DO NOT ENTER, but contact the Auror office immediately.
-- Ministry of Magic, from the leaflet "Protecting Your Home and Family Against Dark Force" (HBP3)
The Dark Mark spell conjures an immense glowing skull in the sky, comprised of green sparks. There is a snake coming out of the skull’s mouth. The spell was used by Death Eaters to announce their nefarious deeds, often displaying it above the scene of a murder.
References from the canon
- This spell is known only to Death Eaters, who send it up in the sky when they kill. The Dark Mark was seen at the Quidditch World Cup in 1994, conjured by Barty Crouch Jr. using Harry's wand (GF9)
- Horace Slughorn forgot to set a Dark Mark over the house where he tried to fake his death (HBP4)
- It appeared over the shack where the body of ex-Death Eater Igor Karkaroff was discovered (HBP6).
- During the Battle of the Astronomy Tower, the "glittering" green Dark Mark hovered overhead (DH27)
"mors" L. death + "mordere" L. to bite
While not definitively stated in canon, the Dark Mark seems to have been invented by Voldemort (or Tom Riddle) as an homage to his revered ancestor, Salazar Slytherin, one of the four founders of Hogwarts. As a young student, Tom Riddle discovered the entrance to the Chamber of the Secrets containing a giant statue of Slytherin from whose mouth the giant basilisk emerged (CS17). Tom spoke to the basilisk, telling it to kill, and thus he murdered Myrtle Warren in the restroom, using her death to turn his diary into his first Horcrux .
To Voldemort the Dark Mark perhaps symbolized the history of Slytherin House, the ability to speak Parseltongue, and more darkly, the power to hunt down victims and kill them. It certainly came to be feared and loathed in the rest of Wizarding society during Voldemort's first and second reigns of terror.
As the Dark Mark hovered over the Quidditch World Cup, Molly Weasley assumed her family had been killed by Voldemort's followers, the Death Eaters. In fact, she was so upset when her husband and children arrived home safely that she had to drink Ogden's firewhisky to calm herself (GF10). To some readers, Molly's reaction seemed overwrought and hysterical, especially when she was usually a strong character. But we have to look deeper into her family history to understand what she was feeling.
Her two brothers, Gideon and Fabian Prewett had been killed during the first war with Voldemort by the Death Eater Antonin Dolohov (PS1, OP9, OP25). Chances are the Dark Mark appeared in the sky when they died, perhaps directly over the Prewett house. After learning that Dolohov escaped from Azkaban with several other Death Eaters, Molly had a moment of post-traumatic stress, suddenly unable to protect herself from a boggart at Grimmauld Place, and viewing her worst fear -- her children and Harry dead one by one (OP25). The Dark Mark unfortunately left a lasting mark on her, making it even more satisfying that she was able to kill one of the worst Death Eaters who ever lived, Bellatrix Lestrange (DH36).