Basically, your Ancestor with a golden voice ruined everything, now you need to fix everything with your army of mercenaries, except everyone has PTSD and your favorite character is always the first to die.

In a more serious note, Darkest Dungeon is a turn based RPG with a focus on teambuilding and positioning. You build teams of up to 4 heroes to battle groups of up to 4 enemies, while making your way through long dungeons filled with many traps and treasures for you to interact with.

Uniquely enough compared to other similar turnbased RPGs, is that rather than just managing Health in combat, you also have to manage a mechanic known as Stress. Stress is effectively a second health bar, and it slowly builds up as your heroes go through stressful experiences. Running around in pitch black darkness, being screamed at by ravenous cultists, attacked by strange monsters, and going through near death experiences are all some of the many ways a hero can build up Stress. Once a hero becomes overly Stressed out, one of two things can happen.

Either the stress can snap their fragile minds, turning them crazy with Afflictions such as Paranoid, Irrational, or Abusive. A Paranoid hero might refuse to be healed by their teammates, thinking it's a trick to stab them in the back. An Irrational hero might decide to attack a teammate at random, while babbling incoherently. An Abusive hero might insult his or her teammates for every mistake, stressing them out even further.

Alternatively, heroes may become Virtuous, a rare and powerful buff, as Adversity can at times foster Hope and Resilience. Virtues can include things like Courageous, Focused, or Powerful. A Courageous hero might encourage their teammates, relieving their stress while being much more resilient to stress themselves. A Focused hero is a lot less likely to miss, and can buff their allies accuracy as well. A Powerful hero gains a large damage buff, etc.

It's a tough balancing act of managing both the Health and Stress of your heroes, as letting your heroes get overly stressed risks Afflicting them and making everything much harder for your remaining sane heroes, meanwhile neglecting their own health will lead to the heroes dying permanently.

Outside of combat, you also have to manage a limited inventory system, juggling food, provisions, and loot while being forced to make tough decisions of what you can do with or without. You also need to manage your heroes quirks and diseases. Quirks are minor stat buffs and debuffs that are gained overtime by your heroes throughout their adventures, though some Quirks can cause negative behavior patterns in your heroes(like making them steal loot from chests and the like before you can loot it). Meanwhile diseases can be inflicted through certain enemy attacks or interacting with certain items. Diseases tend to be mainly negative, offering a lot of major penalties if left unchecked, like heavily reduced speed or health.

The game has a pretty great atmosphere, both artstyle and sound, a lot of interesting ideas, along with having a fun combat system and a wide variety of viable options for teambuilding. With a varied cast of heroes, all of which fully capable of carrying you through the game, you've got plenty of options for how to make your way through the various dungeons and bosses. The combat is RNG-based, in the sense that attacks can land or miss based on the roll of the dice, along with a lot of other things, however Darkest Dungeon provides all the means to counter-act and even negate most RNG with good planning and well thought out teambuilding. It's very rare that bad luck can genuinely screw over the player, and the majority of the time anything going wrong is the player's fault first and foremost.

The game also has mod support, and an incredible modding community, helping add plenty of longevity to the game if vanilla starts to bore you.

However, as with all games, Darkest Dungeon does have it's own set of flaws. The very early-game tends to be a lot more RNG-reliant than the mid-to-late game, giving you far less tools and characters to work with. Some mechanics can be poorly explained or just not explained at all despite being relatively important to the player(such as how Speed rolls work or the Antiquarian's bonus effects). And while there's plenty of different enemies in the game, fights can tend to feel pretty samey, and shared enemies between regions can give the feeling of a lack of enemy variety at times.

I think that should cover everything.