A Dark Lord is a type of villain in a story that seem simply too over-the-top. Dark Lords tend to have a simple goal, like "take over the world" or "kill the hero". While those goals don't always make him/her a Dark Lord, the true sign of one is that it will typically be unclear how they benefit from a world that's theirs or having the hero dead. They also tend to freely acknowledge the fact that they are evil, and actually embrace that trait in certain cases. Dark Lords are usually frowned upon since they don't seem to have any motives and are too unrealistic. No one thinks of themselves as evil.