The definition I prefer is from Wikipedia:
“Grimdark is a subgenre or a way to describe the tone, style or setting of speculative fiction (especially fantasy) that is, depending on the definition used, markedly dystopian or amoral, or particularly violent or realistic.”
Sure it's dystopian in nature, yes it is violent and characters' morality operates in the grey spectrum. But for me, there is an extra ingredient that this definition ignores. It centres on WHY these three components merge. I think the missing ingredient is the context - survival. The lack of morality and the violence, the nihilism that is provoked, are driven by people's need to survive.
I'm not talking about the main characters either, everyone (in one form or another) hangs by their fingertips to Life. To illustrate my point, look at Joe Abercrombie's stories where life has little value (in the First Law trilogy death is inconsequential, torture is acceptable, murder is neither here nor there). Mark Lawrence's work highlights the same, no one is safe, death is always just around the corner. Richard Morgan's fantasy novels epitomize grimdark - Ringel Eskieth has no desire to live, wastes life and dispenses justice with his blade.
And that last point is a second crucial element - within the darkness there is light. People do things which precipitate an outcome that is right. Justice is seen to be done. The truly evil are killed. Life, such as it is, can continue and perhaps offer some hope of improving. Eventually.
It's why I enjoy grimdark stories and prefer to write them. We see people laid bare, all pretense stripped away, as they struggle to survive. We see the human condition for what it really is. I believe it can echo what we see on the news so often, where hatred has gained a loud and potent voice. Sure, I want my stories to entertain but I also want them to have a message.
In The Knights' Protocol the need for survival provokes persecution, particularly of scapegoats, invariably any minority group that is less able to fight back. Anti-heroes will respond to such evil, albeit for amoral reasons, but they triumph in some capacity. They suffer terribly in the process but this only reinforces that, deep down, there is goodness there. It may be discoloured, grey rather than white, shadowy rather than light, but it is there. For me, that insight into the human condition is why Grimdark is a significant addition to the Fantasy genre.