Grimdark fantasy is just as described. It is both grim and dark. The worlds usually have a dark ages feel to them, where life is hard, survival a struggle, cards are normally stacked against you, and prejudices run deep. There is usually foul language, graphic violence, and sometimes graphic sexual content (which is why you have to verify you’re of adult age before getting your hands on it). The official term cropped up with Warhammer, but has since been adopted by suppliers of such content, due to the fact that it is as grim and dark and the origin series.