I recently started playing Sunless Sea, and I think it's one of the most amazing games I have ever tried. It's on par with Portal, although it is a completely different genre.
Sunless Sea is a bit hard to describe. It's an exploration game with elements of role-playing. Resource management is critical to survival. There are opportunities to combat pirates and sea monsters. But it is chiefly a game about discovering stories.
When you choose a character background for your sea captain, you get a quest for that captain. If you survive, you win! Good luck surviving, though...
You can also pursue missions of various kinds, discover mysteries in strange ports of call, and encounter spooky occurrences as your crew descends into a Lovecraftian madness. There are many forces at work in the Sunless Sea, and they range from malign to malevolent.
Yes, I know the words are synonyms.
But the world is also beautifully designed, and there is an atmospheric soundtrack that alternately offsets and harmonizes with the gothic horror. The artwork is superb. Everything about the world feels right even though everything in its alternate history went very wrong.
This game probably won't appeal to anyone who plays only fast-paced first-person shooters, but it's great for those who play role-playing games, real-time strategy games, world-building games, or other deep games with lots of opportunity to experiment. It's incredibly atmospheric and immersive, although it is also quite difficult. I can't recommend it highly enough though.