Grimdark is a fantasy subgenre that is particularly dystopian, amoral, or violent. Its protagonists tend toward the conflicted antihero, and its worlds are often brutal and nihilistic. The writing also tends to be much grittier and realistic. A whimsical quest to get the magic doohickey this is not.
The slightly overwrought term is inspired by the tagline of the tabletop strategy game Warhammer 40,000: “In the grim darkness of the far future there is only war.”