Muggles' Guide to Harry Potter/Magic/Morsmordre

Overview
Morsmordre is a spell that conjures the Dark Mark.

Extended Description
The Dark Mark symbolizes very black magic. When cast, this spell produces a large, luminous green skull that hangs in the air. This is Lord Voldemort's sign, which the Death Eaters used to leave as a calling card over a site where they had worked their evil.

Analysis
The Dark Mark is actually the name of the sigil: a skull with a snake twisting through it. Death Eaters are tattooed with this mark on their left forearm. Morsmordre is a spell that produces a form of the Dark Mark, made of glowing green particles, that hovers in the air.

Morsmordre is a spell known only to Death Eaters, and the appearance of the Dark Mark in the air over the Quidditch World Cup is a clear indication that there is at least one Death Eater still at large and still willing to be identified as such. This causes immediate panic on the part of the crowd, who all remember times when that mark was left floating above a house that had been destroyed.

After Voldemort's return, we would expect to see it more often; however, it does not reappear in Harry Potter and the Order of the Phoenix. During this time, Voldemort is secretly rebuilding his power, aided by the Ministry policy of denying his return. Having the Dark Mark appear would, of course, decrease the chance of remaining undetected – though it is likely the Ministry would simply try to spin things to avoid saying Voldemort had returned.

The Dark Mark reappears several times in Harry Potter and the Half-Blood Prince, where, as before, it is used to mark areas where Death Eaters have been working their evil. It is specifically noted as floating over the hut where Igor Karkaroff was found murdered, and over Hogwarts during the invasion by the Death Eaters.

A Latin etymology is possible: mors, meaning death, and mordre, from the vulgar Latin mordere, "to bite." This presumably refers either to the snake present in the image, or was more euphonic than the verb "to eat"; mors mordre - a Latin approximation of Death Eater.

Morsmordre as one word and not broken up is actually (intentionally or not) Norwegian. Directly translated it means "motherkillers". This symbolizes the kinds of persons the Death Eaters are, they could kill their own mothers for the sake of Voldemort and purity of the race.