Final answer:
In the Harry Potter series, the Dark Mark is a symbol used by Lord Voldemort and his Death Eaters to mark their presence and to summon their followers. It appears in the form of a skull with a serpent emerging from its mouth in the sky.
Step-by-step explanation:
In the Harry Potter series, the Dark Mark is a symbol used by Lord Voldemort and his Death Eaters to mark their presence and to summon their followers. When a wizard is being summoned by the Dark Lord, the Dark Mark in the form of a skull with a serpent emerging from its mouth appears in the sky. It is described as emitting a green light. The Dark Mark does not burn with a black flame, turn icy cold to the touch, or disappear temporarily as mentioned in the options.