Harry Potter and The Stolen Plot...

[Gayalondiel]

Dark Lord...jeez, she didnt even try to change that one...

I feel i should point out that, like so many things already listed here, the term "Dark Lord" was not created by JRRT, nor is it specific to his work. Traditionally, the forces of evil have always been associated with darkness and the term is pretty generic for the chief bad guy of most mythologies. (Another figure who claims the name is this guy called Satan, I think he's been around a bit longer than Sauron)