The term and similar concepts enjoy widespread popularity as a stock character and a villainous moniker in fantasy and related genres as well as in literary analysis of such works.
As the name implies, a dark lord is characterized as a given setting's embodiment of evil, darkness, or death (either metatextually or as literal figure within a work's mythopoeia) in a position of immense power, most often as a leader or emperor with a variety of minions and/or lesser villains at their disposal to influence their conflict against a heroic protagonist in a primarily indirect way, though they may additionally be depicted as wielding great physical or magical capabilities should a hero ever confront them personally.
Later The Silmarillion would focus on the character of Morgoth, of whom Sauron was the principal lieutenant and then successor, while other works would further popularize and diversify the concept with antagonists such as Darkseid in the DC Universe, Emperor Palpatine in the science fantasy Star Wars saga, or Lord Voldemort in the Harry Potter series.
More recent works sometimes also move away from the archetype's mythic origins in favour of historical allusions to infamous conquerors and dictators such as Julius Caesar or Adolf Hitler.
[1] The Encyclopedia of Fantasy notes that common features of a dark lord character include being "already defeated but not destroyed aeons before" and engaging in "wounding of the land" or other rituals of desecration.
[11] In Australia, a minor literary scandal erupted in 1997 when it emerged that award-winning author Helen Darville plagiarised the list for her regular column in Brisbane's The Courier-Mail newspaper, which led to her being fired.