In J. K. Rowling's "Harry Potter" books, the Death Eaters had a magical tattoo called the Dark Mark. Each Death Eater bore this mark on his left inner forearm.
The Dark Mark appeared to be a skull with an intricately self-entwined serpent coming from its mouth. The Mark appeared spontaneously on those who gave themselves over to Voldemort. Death Eaters and others who had carried out Voldemort's evil deeds could summon a similar magic sigil using the special spell. This spell created a glowing ethereal Dark Mark that floated over a victim or the area where the deed had taken place. Voldemort used the Dark Mark tattoo as a means to summon or be contacted by his Death Eaters.