Dark Mark

The tattooed shape of a skull with a snake crawling out of its mouth with which Lord Voldemort branded the left forearm of his followers.

• Harry refers to the Dark Mark tattoos as "scars" that are burned onto the arms of the Death Eaters (OP19). Harry knew exactly how they worked from watching the Dark Lord use Peter's Dark Mark to call the Death Eaters in the graveyard at Little Hangleton (GF33).
• Snape told Fudge the Dark Mark is "burned" into the skin of a Death Eater by the Dark Lord, and when one scar was touched, each of them felt pain and knew to apparate or disapparate to the location of Lord Voldemort (GF36)
• The Dark Mark is a type of Protean Charm, same as the changing coin Hermione Granger used for Dumbledore's Army (OP19). In fact, she got the idea from Voldemort, but only the coins burned hot, not the skin of the members.
• During the year that Peter Pettigrew helped nurse Lord Voldemort back to life, the Dark Marks of the Death Eaters became visible and painful again. Igor Karkaroff becamse so frightened by his Dark Mark, he fled Hogwarts rather than face Voldemort again (GF23, GF35).
• The use of the Dark Mark must have been a well-kept secret. When Harry asked Sirius Black why Karkaroff would show Snape something on his arm, Sirius Black said "I've no idea what that's about," even though he had spent 12 years in Azkaban among the Death Eaters (GF27).
• Cornelius Fudge, Minister for Magic, also apparently didn't even know Dark Marks existed, and was in denial about Harry's scar being a harbinger of Voldemort's return.  He said: "You'll forgive me, Dumbledore, but I've never heard of a curse scar acting as an alarm bell before. ..." (GF36). Snape stepped forward and showed him his Dark Mark, telling him how it had "burned black" when Voldemort used Peter Pettigrew's Dark Mark to call them. However, Fudge never believed any of it until Voldemort appeared at the Ministry of Magic a year later (OP36).
• Bellatrix berated Snape for not returning to the fold of Death Eaters the minute the Dark Mark burned. Snape replied that he had to be more careful around Dumbledore to protect his position as a spy for Voldemort, and he returned two hours later anyway (HBP2).
• Draco Malfoy became a Death Eater some time before his sixth year at Hogwarts. He showed his Dark Mark to Mr. Borgin in Diagon Alley in order to intimidate him (HBP6).
• Narcissa Malfoy never took the Dark Mark, unlike her husband and son, even though she shared their pureblood racist views. However, once the Dark Lord threatened Draco's life, she changed her mind (BLC).
• Bellatrix used her Dark Mark to summon the Dark Lord to Malfoy Manor as soon as she knew the Snatchers had caught Harry Potter (DH23). But she acted too soon because Dobby the house-elf arrived and rescued all the hostages including Harry (DH24). When Hermione was undercover as Bellatrix at Gringotts, another Death Eater named Travers taunted her: “I heard that the Inhabitants of Malfoy Manor were confined to the house, after the . . . ah . . . escape” (DH26).

The Dark Mark seems to have been invented by Voldemort (or Tom Riddle) as an homage to his revered ancestor, Salazar Slytherin, one of the four founders of Hogwarts. The snake coming out of the mouth refers to Slytherin's ability to speak Parseltongue, the language of snakes (Pm, CS11).

The spell "Morsmordre" puts a giant green Dark Mark in the sky over a place where Death Eaters have terrorized people (GF9). But on the arms of his Death Eaters, the Dark Mark appears black (OP36).