One of the important themes of the Harry Potter novels is the idea of good versus evil, personified by the battle between Harry Potter and Lord Voldemort. Each individual has a following of individuals that support his cause: in the case of Harry Potter, these individuals comprise most of the wizarding world, fighting to avoid takeover by Voldemort. Lord Voldemort, however, has a select band of witches and wizards known as Death Eaters to carry out his wishes and support his aim of eventual domination of wizarding society. In order to be considered a Death Eater, someone must be loyal to Lord Voldemort, and willing to commit acts of terror on his behalf. Furthermore, once accepted as a Death Eater, an individual cannot remove him or herself from the organization except by death.
The exact number of people known to be Death Eaters is unclear in the books, although it sounds like a small group. Lucius Malfoy is known to be a Death Eater, along with other members of pure blood families, and many children of Death Eaters attend Hogwarts. Many of these children are in Slytherin House, but J.K. Rowling has indicated that other houses may contain the children of Death Eaters as well, suggesting that Voldemort's organization may be extremely widespread. There are also indicators that the Death Eaters may be a worldwide organization, given the membership of Igor Karkaroff, and the foreign names of some other Death Eaters.
It is assumed that a wizard or witch must be a pure blood in order to be a Death Eater, because Voldemort wants to eliminate families with muggle heritage, even though his own father was a muggle. Certainly, most of the Death Eaters take pride in being pure blood wizards, and look down upon wizards of mixed blood. It is also suggested that being a Death Eater is accompanied by being evil, because the Death Eaters regularly carry out acts of terror including murder and torture for amusement. Many muggle killings are linked with the Death Eaters, who are seen tormenting a muggle family in Harry Potter and the Goblet of Fire. Most of the Death Eaters have also performed Unforgivable Curses, and have spent some time in Azkaban, the wizarding prison, as a result. A certain twisted sensibility may be required in order to be a successful Death Eater.
The Death Eater uniform appears to be a hooded black robe and a white or silver mask obscuring the upper portion of the face. In addition, every Death Eater wears a brand of the Dark Mark, Voldemort's symbol, on his or her arm. Voldemort can use the Dark Marks to communicate with his followers, as they will turn black when he requires their presence. The Dark Mark combines two symbols associated with Voldemort: a grinning skull and a snake, and is also used to mark homes which have been visited by the Death Eaters, in the form of a large illuminated Dark Mark which will hover over the location.