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 spell, even though, beyond its symbolic impact, there's no indication the spell causes any physical damage.