The Venetian habit of wearing masks originated with a group of debt-ridden nobles. To avoid being recognized by their creditors and enemies, they wore masks and cloaks as they moved through the streets and alleys of Venice.
Later, some people wore elaborate masks to secretly meet their lovers. They rowed small boats beneath their lovers' windows and climbed up the ropes lowered to them. Besides these thieves of hearts, there were also thieves of jewels. The custom even spread to Spain, then was carried by the Spanish to South America. The famous swashbuckler "Zorro" was also a masked man. It was an Italian-French co-production about the people rising up against a governor's tyranny.
Before the eighteenth century, masks were part of Venetian life. The law even allowed Venetians to wear masks while working and living for most of the year, until Napoleon invaded Italy and conquered Venice. From then on, masked figures vanished without a trace.
Death Eaters also wore masks. Their masks were black, covering most of the face and leaving only the chin exposed. They bore subtle patterns and looked exceedingly ornate.
But as Philippe had said, they were "masks of death." With those masks, even if a victim saw the murderer up close, they still could not identify them unless the killer removed the mask. That was why Karkaroff and Lucius Malfoy had made their bargain: by identifying other Death Eaters in hiding, they cleared themselves of guilt and avoided Azkaban.
Bellatrix never wore a mask. She was a loyal follower of the Dark Lord and felt no shame whatsoever in being a Death Eater.
After Voldemort's two defeats, Death Eaters became notorious. Even those who had once joined them merely by following the crowd desperately concealed that part of their past, and the Dark Mark on