Basic Mysteries of Life: Eldritch
Eldritch is a terrific little game. Drawing from the Lovecraft mythos, it gives you procedurally-generated levels full of increasingly-terrifying monsters to sneak, fight, and explore your way through. It doesn't bother with story: You wake up in a mysterious, infinite library with a handful of glowing books. Opening one transports you to a world where you must survive the strange to return to the library with one of the souls of the gods of that place. Return all three, and you unlock a mysterious door in the library.
Monsters are fish-people who glub and robed wizards who grunt. They react to sounds, which means they can chase down your gunshots or run toward that rock you just threw. Mix in the Minecraft aesthetic and destructible terrain (mostly through dynamite at first), and later indestructible monsters, and you have a strong recipe for delicious gaming.
It grew after its initial offering, too. Originally with three worlds, the creator added a Halloween world and a mysterious ice-world staffed with invincible penguins, among other monsters. Pickaxes, tripwire guns, hatchets, and more gear also appeared in the expansions.
There's a lot to explore, and while I admit I don't have the patience to start another game after dying this close to beating the third world and having to start over, it's a game I'll return to from time to time.
You can buy Eldritch on Steam or on its own site for $14.99.