Dwarf Fortress, the ultimate dwarf simulator

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.