Commit a dark crime, cast one of the three unforgivable curses, mutter morsmordre, and the dark mark will appear to let the world know what you’ve done.
Every one of Voldemort’s Death Eaters bear the dark mark somewhere on their body, so why not you?
A Dark Mark tattoo will let the world know that you think it’s fun to do bad things. The symbol of a snake emerging from the mouth of a skull is downright heavy metal and it will unite you with other Death Eaters around the world.
The mark first appeared to Harry and Ron in the beginning of The Goblet of Fire after Barty Crouch Jr. catches some Death Eaters having fun with unsuspecting muggles. At the time, he did it to scare Voldemort’s former followers into thinking that He Who Must Not Be Named himself had returned. Because they betrayed their former master, they fled, but not before the entire crowd dissolved into panic-stricken chaos. Little did they know that their master would assume flesh and blood before the second semester was over.
People who bear the Dark Mark know that honor and moral codes are just false constructs imposed by those in power to control the masses. They don’t submit to these false values anymore than they submit to institutions or abstract notions of good and evil. In this world, there is only weakness and power. Bear the Dark Mark yourself and let the world know which side you chose.