In "religious orders" (as opposed to entertainment literature) contexts, it usually means Satan or other similar entities who hold power over lesser fiendish creatures and seek to disrupt the comfort and lives of people, sometimes tragically, and definitely maliciously.
In fantasy novels Dark Lords have become something of a cliché following the success of J. R. R. Tolkien's The Lord of the Rings, in which the Dark Lord Sauron is a disembodied force, sometimes visualised as a lidless all-seeing evil eye.
Following the example of Sauron, Dark Lords in fantasy are always depicted as immensely powerful, implacably evil creatures with an insatiable lust for power, who cannot be reasoned or bargained with. Only by their ultimate destruction can peace be restored.
Among the Dark Lord's in fantasy and science fiction are: