Morsmordre is the incantation of a curse used to create the image of the Dark Mark.
When casted, it leaves a large, faintly translucent, wisp-like, hazy green skull with a serpent coiling out of its mouth in the sky. The Dark Mark announces its presence upon creation by playing a roar that can be heard by every player in the server.
Only one can exist in a game at once, and there is a cooldown between casts of Morsmordre. Uniquely, this spell does not cast exactly where one aims toward.
Morsmordre is a spell also taken from the series Harry Potter, which also uses the same Dark Mark, the symbol of Lord Voldemort and his Death Eaters.