Dark lord

The Dark lord is a stock character in fiction and mostly fantasy who is normally an evil villain who is very powerful.
Origin of the Phrase.
In the Middle Ages and Renaissance, when people talked about The Devil, they called him The Prince of Darkness, the Lord of Darkness or Satan. So when John Milton wrote his book Paradise Lost which had Satan as the protagonist he called him Satan, Lord of Darkness and called him the Dark Lord (though he usually just called him Satan.)
C.S. Lewis.
In his Christian science-fiction novel Out of the Silent Planet, C.S. Lewis had each planet be ruled by an alien space angel called an Oyarsa who was sent there by Maleldil son of the Old One. But the Oyarsa of Earth tried to start a rebellion against Maleldil so he was banished to Earth and while there tried to make everyone evil to destroy Maleldil’s laws so Maleldil came to Earth and became human to stop the Fallen Oyarsa to the point where Maleldil died to save the world from him. In the sequel Perleandra, the narrator calls the Fallen Oyarsa the “Dark Lord of the Silent World” and the Fallen Oyarsa turns out to be The Devil.
J.R.R Tolkien.
In the epic fantasy novel The Lord of the Rings by J.R.R. Tolkien there is an evil wizard named Sauron of Mordor who creates a magic ring that makes everyone who wears it evil as part of a plan to take over Middle-earth. Sauron was the first character in a fantasy novel ever to be called The Dark Lord.
Star Wars.
In George Lucas’s Star Wars movies Emperor Palpatine is known as the Dark Lord of the Sith.
Discworld.
In The Last Hero, Terry Pratchett parodied the trope of The Dark Lord with the character of Evil Harry. 
Voldemort.
In J.K. Rowling’s Harry Potter books Death Eaters call Lord Voldemort “the Dark Lord. 
Evil Overlord List.
In the 1990s, someone named Jack Butler, on his blog made a list of mistakes that Dark Lord characters usually make which he wouldn’t make if he were a Dark Lord called the Evil Overlord List. Since then many different people have added different things to the Evil Overlord List.