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.)