Darkest Dungeon is a turn-based RPG, in which you assemble a team of four heroes each week to delve into a dungeon and fight the horrors inside it. These heroes become stronger as they overcome these horrors, but if they are unable to overcome them, they will be killed. Each adventurer you get is unique. There are many different classes they can be, each with their own skillsets, and each adventurer gets a set of quirks that set them apart from the others. Quirks can be positive or negative, and can be changed as time goes on, but these quirks make heroes with the same class still feel different.
Darkest Dungeon has roguelite elements, which means that when heroes in the game are killed, they are killed permanently. This means that you need to play carefully and conservatively if you want your adventurers to live.
The interesting thing about Darkest Dungeon, and the reason why I like it, is that it focuses not just on the physical dangers of dungeoneering, but also the mental ones. On top of managing your party's Health, they also have a stress bar that you need to keep in check. If your adventurers get too stressed out, they will go insane, getting an affliction that makes them weaker and stresses out other party members. If they get stressed too far after that, they will have a heart attack and die. This idea of including the mental dangers of dungeons is what got me interested in the game, and after playing it I fell in love.