What work originated the term "dark lord" as a conventionalized title for a preternatural overlord?

Question

In a comment on one of my recent answers, someone snarked at my calling Darth Vader "the dark lord," as if that title were specific to Voldemort from the Harry Potter novels. Actually, the title is much older than Rowling's novels, and has been particularly associated with a number of other characters from fantasy and science fiction. In modern fantasy, it is not at all unusual for a sorcerous overlord to be known by the appellation "dark lord" (or some variant on that). Before Voldemort, the most famous example was Sauron, from The Lord of the Rings, who is frequently referred to as the "dark lord," although he has many other epithets as well: "black lord," "the enemy," "necromancer," "Gorthaur the cruel," etc. However, I am wondering about the history of the title before Tolkien.

A Google Ngram plot showing the frequency of "dark lord" in the Google Books corpus shows sporadic appearances up to about 1940. Then, after 1950 it begins a gradual increase, which can probably be attributed to the growing popularity of J. R. R. Tolkien's works.

Then there is a rapid increase in appearances in the 1970s. Some of this is likely still due to increasing references to Tolkien, but there are probably also other fantasy works, and (by the end of the decade) Star Wars. (The second big increase, in the 1990s, is most likely driven by references to Harry Potter, which.)

In the original Star Wars novelization, Vader's title when he first appears is given as "Lord of the Sith." However, it was always commonplace to refer to him as a "dark lord"—so much so that when, in the run-up to The Phantom Menace, it was revealed that the other Sith in the movie would also have names starting with "Darth," people immediately started joking that "Darth" was short for "Dark Lord of the Sith." In fact, the name "Darth Vader" had always been a play on "Dark Father" (even before George Lucas decided definitively that Vader was Luke's father).

So there are number of characters from speculative fiction that have been characteristically known as "dark lord," but who was the first? The phrase does appear (at low levels) before the publication of The Lord of the Rings, so—as influential as that trilogy may have been in terms of popularizing the term (as well as the dark lord character type itself)—it does not seem to be the first. And is it possible to tell whether the earliest usage actually influential, or did later authors like Tolkien effectively reinvent the "dark lord" title?

(I identified a possible source, through the research I was doing while writing the question. I will add that as an answer below. However, it involves a work I am not familiar with, and one that I found after only a relatively cursory search. So my self answer should by no means be considered dispositive.)

Answer 1

I believe that it didn't originate in a work of fantasy, but rather in religion. Concretely, in Christianity, Satan was called the Prince of Darkness. This appears in Paradise Lost, by John Milton, and is thought to derive from the Latin term princeps tenebrarum. This latter term, however, is sufficiently ambiguous, despite being the origin of the modern word "prince," that "Lord of Darkness" would probably also be a reasonable translation. The form "dark prince," grammatically closer to "dark lord," can be found in a number of 19th-century religious texts. The form "dark master" is also well-attested. "Dark lord" would not have been used much, since the term Lord in religious contexts was reserved for God, but it is a logical extension. It seems that "dark" + any term of rulership was a common periphrasis to indicate Satan. Therefore, although I can't find a book in the digitized corpus that uses the phrase to refer to Satan before the 20th century, I strongly suspect that it was first used to refer to the Devil.

As a side note, the Latin term in turn likely comes from Manichaeism, which had a highly dualistic darkness/light cosmology, and where the title "Prince of Darkness" (or the equivalent) was associated with the principle of evil. It is also possible to find the term "Lord of darkness" used to refer to the same deity in some old books. Satan was, of course, often equated with this Lord of Darkness.

Answer 2

I found an occurrence of "dark lord" in The Lord of the Dark Red Star: being the story of the supernatural influences in the life of an Italian despot in the thirteenth century (1903) by Eugene Lee-Hamilton. The precise phrase "dark lord" only actually occurs once in the book, spoken by a witch addressing the devil:

"O Lord, dark Lord," she said, "I bring the blood thou claimest, even that of Ezelin's first born. When, like a bat, the unbaptised soul shall flutter up for thee to seize, then thou wilt fill the measure of my desire, and make my son a king. But first thy mark must be upon it."

However, both darkness and baleful evil lords (both temporal and ethereal) appear to themes of the novel—as is clear from the title alone.

This is not the earliest attestation of the phrase, however. The most prominent place where "dark lord" shows up before 1903 that is in Tennyson's 1889 poem "Demeter and Persephone." The last stanza includes:

Yet I, Earth-Goddess, am but ill-content
With them, who still are highest. Those gray heads,
What meant they by their "Fate beyond the Fates"
But younger kindlier Gods to bear us down,
As we bore down the Gods before us? Gods,
To quench, not hurl the thunderbolt, to stay,
Not spread the plague, the famine; Gods indeed,
To send the noon into the night and break
The sunless halls of Hades into Heaven?
Till thy dark lord accept and love the Sun,
And all the Shadow die into the Light,
When thou shalt dwell the whole bright year with me,
And souls of men, who grew beyond their race,
And made themselves as Gods against the fear
Of Death and Hell....

I have quoted quite a bit on either side of this appearance of "dark lord," to make the context in which Tennyson used the phrase apparent. It seems, in this instance that "dark lord" is just a description of Hades, the god of the underworld who abducted Persephone—and who is frequently associated in Greco-Roman iconography with the color black. So Tennyson's application of "dark lord" simply appears to mean "a lord who is dark and/or prefers the darkness"; it does not seem to denote a standardized title.