I’ve seriously debated posting in this space, as evidenced by the lag time since the last post.
As someone above said, folks can get kind of shrill on the matter …
At any rate, I think I’d like to take a pass at answering the OP’s question.
But first, let me assure, all that should sparkle in either genre is the prose and cold steel (or blood) in sun/moon/torch light.
The dark fantasy Wikipedia definition above is okay, if a bit ambiguous, so I’d like to take a stab at (over)simplifying it: elements of supernatural horror set within a fantasy milieu
It seems a shame that grimdark doesn’t have its own topic; since I do think they are two very different things. If anyone knows someone suitable with edit rights, give them a poke?
Dark fantasy (all IMO) - the Dark tends to be external, often in the guise of a (supernatural) Other and/or any who serve it.
And with the Dark, there will be the Light to oppose it. It may not always be the bright, white Light of the pure Chosen/Savior, but whatever their moral hue, the protagonist(s) see(s) that the Dark is Evil, and seek(s) to actively confront it.
And for dark fantasy, I think ‘seeing’ is the key here. What sets it apart from similar works of epic or high fantasy is that the reader sees the Evil on the page, and the protagonist(s) experience(s) the Dark personally, and they (and the reader) know what side they are on.
((As a contrasting example for epic/high, we take Tolkien’s word that Sauron is evil, but we don’t really see him do any directly. Sure, he and the Nazgul fit the (supernatural) bill, but mostly he sends his legions out to conquer the world, a thing many mortal men have done, some in the name of good. ))
Grimdark fantasy - the dark tends to be internal. I’ve altered case here to reflect the relative position of the word ‘dark” within the two phrases, because I think its emphasis (or lack) helps illustrate the difference between the two.
The protagonist(s)/antagonist(s) have borne witness to the evil that folks do to one another. They (and the reader) see the varying moral shades of the characters and it colors their perceptions. Sometimes the reader can’t be sure what side the character is on. Often the characters aren't even sure themselves, and who is fit to render that judgment? Many around them seem dark as well, and light may be in short supply.
This may also affect the tone of either type of work.
In dark fantasy, hope springs eternal, however long forlorn. In the end, the reader has faith that, even if the protagonist(s) has fallen to the Dark they will find redemption, and Light will prevail. And that is usually so.
In grimdark fantasy, hope may be in short supply and the reader may be uncertain who will (or should) prevail. And, the character(s) may seem beyond redemption, but if they do find it, it will seem all the more sweet being unlooked for.
And ... I think I will stop there, in hopes of additional opinions.
And FWIW, had the phrase been in literary use at the time, I think KEW might have called Kane’s stories grimdark, but I wouldn’t say the same of Moorcock and his Elric stories.
