Perhaps most prevalent in works of fantasy, the dark lord (or lady) is exactly what it sounds like: an evil overlord bent on dominating the world (and/or galaxy) who will stop at nothing to get what he wants. He rules his forces with an iron fist of terror and his reign means oppression and misery for the freedom-loving denizens of the realm.
Often the protagonist of a story will be tasked with preventing this rise to power, or toppling the cruel regime if he's already secured the throne. This task usually proves beyond daunting due to the seemingly infinite power and military might at the dark lord's command.
It's not hard to see where this trope likely comes from. Our history books are filled to the brim with the names of conquerors, kings, emperors, and dictators that made their marks on the world via brutal, tyrannical means--marching armies across the countryside in bloody campaigns of conquest, stifling the liberty of the very citizens they swore to serve, and plunging the world into war. It's only logical to conclude that these larger-than-life figures have influenced the stories we've been telling each other over the years, from the fables and folklore of centuries past to modern bestsellers.
The dark lord is such an obvious metaphor for real world despotism that some authors have actually had to go out of their way to deny seemingly overt symbolism in their work, including fantasy titan J.R.R. Tolkien, whose use of the trope has undeniably influenced countless works that followed.
In Tolkien's seminal work, The Lord of the Rings, a humble halfling must prevent the return of the titular character, the dark lord Sauron, who had previously brought war to Middle-earth in his quest for dominance.
The story seems (by some) to be a metaphor for the Second World War, with Sauron standing in for Hitler, and other characters or groups in the book symbolizing various players in the war depending on who you ask (my personal favorite has the Ents playing the role of America). Tolkien firmly denied this theory in the foreword to the second edition, leaving history aficionados wanting.
The primary universe that my own fantasy stories have taken place in thus far (one of which will be seeing publication soon--stay tuned) definitely has its own share of tyrant kings and crooked emperors. My two biggest influences when it comes to fantasy are probably Tolkien and George R.R. Martin. However, the power struggles and political machinations in my "forged realm" are definitely more akin to Martin's War of the Roses-inspired Game of Thrones than to Tolkien's mold, so I don't think I have any genuine dark lords to my name just yet.
That isn't to say that I don't enjoy a good old fashioned evil overlord. I've never been one to balk at the use of tropes like this one (as this series has no doubt shown by now), in fact, I quite enjoy them when handled well. And there's nothing quite so compelling for a sympathetic hero as a nigh unstoppable foe bent on world domination. So keep the dark lords coming, I say.
The Lord of the Rings by J.R.R. Tolkien
The Chronicles of Narnia by C.S. Lewis
The Sword of Truth series by Terry Goodkind
Final Fantasy VI
The Legend of Zelda