The word is "grimdark", and these days it's often followed by the word "fantasy". The logic behind producing such work seems to go like this: Life is cruel, therefore any story with copious cruelty is true to life, and the more cruelty the closer the story hews to truth. I don't think the author believes our world is like that, only the world they've created, but it still strikes me as being awfully narrow.
Some of the cruelest and most unforgiving works of fiction in memory have been favorities of mine because they know better than to think the display of cruelty is the sum total of the ambition. Most everything Hubert Selby Jr. wrote was rough going, but also tinged with great and tragic compassion for his characters. Gary Oldman's sole effort as director, Nil By Mouth, is so hard to watch I almost turned it off halfway through, but redeemed by how it sees more in its story than just how people can be mean to each other and themselves.
There used to be a mood in fiction — to some degree in the mainstream, but mainly in the avant-garde — that the more brutal you were to the reader, the closer you were cutting to the unvarnished truth of things. (Again: life is cruel, therefore any story with copious cruelty is true to life, etc.) I was skeptical of this when I first encountered it and reject it entirely now, not only because it makes for awfully dank reading but because it embodies a worldview I also reject.
While I was struggling to read the first volume of Game of Thrones, I kept thinking about this a great deal, because it was clear to me Martin wanted to give the decent folk of his setting at least half a chance to register with the reader. But the cruelty and the misery and the general Jacobean nihilism of it all ended up eclipsing everything else, and I couldn't help but feel this was because Martin felt justified in presenting this as the Truth Of Things in his setting. Things really are terrible, or so the thinking goes, and so by showing you terrible things I am simply being honest about our wretched lot, "fantasy" or no. People who boast about their honesty are among the ones I most reflexively distrust now, as many people wrongly conflate honesty with good faith and allow themselves to be fleeced by the ones billing themselves as "honest".
Jonas Mekas, whom I have disagreed with about a great many things, once said something I agree with completely: the way to drive out the ugly things in the world is not to make uglier things in a bid to "expose" them; it is to make lovely things, so that we might crowd out the ugly ones. (Make Good Art, said Neil Gaiman.) Sometimes it is required to bring the ugly things to light in order to make them known. But it is not required to substitute ugly things for lovely ones in the name of some spurious bid for truth.