Dwarf Fortress is a single-player fantasy game. You can control a dwarven outpost or an adventurer in a randomly generated, persistent world.
What it lacks in graphics it makes up for in complexity. I’ve always been too intimidated to play Dwarf Fortress but hearing stories like “we had to improve cats’ livers because the dwarves were spilling their drinks and the cats were drinking the puddles and dying of alcohol poisoning” puts it up there with EVE or Factorio as something I’d love to have the time to dive into.
While they did a paid Steam/Itch.io version recently with fancy graphics, the original is still free to download.
Just an insane game in general. There’s a “no clip” documentary about it on YouTube that’s really good
My favorite story about the insane depth of simulation is when cats were mysteriously falling dead and they figured out that they were walking on the floor in the tavern, stepping on spilled booze, and then shortly after, licking their paw to clean themselves, giving them alcohol poisoning, and killing them en masse.