Whats Dwarf Fortress?
It's a brutally hard, incredibly detailed roguelike.
It has two modes - Adventurer, which is what is says on the tin, and Fortress, which is what most people play it for.
You start off with seven alcoholic dwarves and a wagon of supplies. Maybe some animals, if you chose them. Your objective is to not die - which you eventually, inevitably, will. Strike the Earth, and remember, Losing is FUN.
It also requires a hideous amount of processing power. Keeping in mind it calculates humidity, air pressure, air flow, water pressure and flow, temperature, if there are pathogens in the air etc on every single space on the map, up and down (100+ up, and I've had a map with 270+ zlevels going down), as well as tracking each living creature down to blood vessels and nerves...
And it looks like this.
Or like this, if you use a tileset.