The Dark Mark is The symbol of He-Who-Must-Not-Be-Named, appears as a skull with a serpent coming out of its mouth like a tongue.
Dark Mark History
He-Who-Must-Not-Be-Named marked his inner circle with the Dark Mark, the Dark Mark was placed on the inner left forearm of the Death Eater as a sign of loyalty. He-Who-Must-Not-Be-Named could then touch the Dark Mark of any Death Eater which would cause the mark to burn and turn black. Alternatively, the Dark Mark could be summoned in the sky using the Dark Magic incantation “Morsmordre,” this was most commonly used by the Death Eaters to strike fear into those who stood against them. During the 422nd Quidditch World Cup the Dark Mark was conjured causing a panic. Also was used to mark homes where the Death Eaters killed the occupants.