So I was rereading Tolkien’s essay On Fairy Stories–because, you know, that’s my job right now–and found he had some very clear thoughts on the subject of grimdark, even though it wasn’t “a thing” during his lifetime.
In paragraph 82 he notes that the richness of our artistic heritage offers a unique danger: that of boredom or an anxiety to be original. This is a danger because it can lead us to despise what has become trite and familiar in favour of the startling and titillating. He writes:
“But the true road of escape from such weariness is not to be found in the wilfully awkward, clumsy, or misshapen, not in making all things dark or unremittingly violent; nor in the mixing of colours on through subtlety to drabness, and the fantastical complication of shapes to the point of silliness and on towards delirium. Before we reach such states we need recovery.”
And that is where grimdark has strayed from the foundation Tolkien laid for fantasy. It has pursued the dark and unremittingly violent in an attempt at authenticity; it has abandoned hope rather than seeking to recover what we’ve lost along the way. And the end result of grimdark is clear: darkness, pain, and despair await. But for Tolkien, the end is Joy, “Joy beyond the walls of the world, poignant as grief” (99).
You see, Tolkien does not ignore the pain and loss and grief that run rampant in this world or in the secondary world he imagined, he just denies the universal, final defeat that nihilism foresees. Why? Because he knew that defeat doesn’t have to be the end of the story. As a Christian, he knew what kind of story he was living in. He knew that, despite the horrors of two world wars that supplied much evidence for the inevitable defeat of hope and peace, the Christian story begins and ends with Joy. New life brackets the story. Birth and resurrection provide the frame. There might be horrible pain filling the story’s middle, but the Joy at story’s end is everlasting. As St. Paul wrote: “For momentary, light affliction is producing for us an eternal weight of glory far beyond all comparison.” And that man knew affliction in his bones.
So hold fast to your hope, even when the darkness seems impenetrable. Day will dawn, and Joy will follow in its wake.
 J. R. R. Tolkien, Tolkien On Fairy-Stories, ed. Verlyn Flieger and Douglas A. Anderson (London: HarperCollins, 2008).